Gareth Clubb called for Neil McEvoy's Adjudication Panel for Wales hearing on 2 and 3 March to be delayed.
Plaid's spring conference is being held in Newport on 3 and 4 March.
The Adjudication Panel for Wales has been asked to comment.
Mr Clubb said the clash "could lead to an erosion in public trust" in the work of the panel.
Mr McEvoy, a Plaid AM for South Wales Central and the party's group leader on Cardiff council, is facing a tribunal over allegations he brought the authority into disrepute in his role as a local councillor.
BBC Wales reported in January that the case, referred to the tribunal by the Ombudsman, apparently relates to whether a comment was made to a council officer in a threatening way.
In a letter to the panel, Mr Clubb said the Fairwater councillor "will be speaking at the conference and we would not wish his speech to be disturbed either by his appearance at a panel or by his having to prepare for an imminent hearing".
He said the case "could have been heard at almost any point in November through February on a date that would not have conflicted with a critical date in the political calendar".
Mr Clubb, who has served as Plaid's chief executive since last September, said timetabling the hearing for the day of the conference gave the "appearance of political interference, even if the date clash is entirely coincidental.
"I fear that scheduling Mr McEvoy's case to coincide with our spring conference fails this test and could lead to an erosion in public trust in the work of the panel."
In the letter Mr Clubb suggests it could be seen "that the panel is trying to unfairly prevent [Mr McEvoy's] appearance at conference".
He said an "impartial decision" would rightly determine that the "proposed date is manifestly unfair to Mr McEvoy".
Mr Clubb, who said the dates of the conference had been public knowledge since November, called for the hearing date to be changed so it avoided the conference and the "formal local election period".
Council elections take place on 4 May.

Salem, the state capital, hired a herd of 75 goats to munch their way through plants like Armenian blackberry in the city's largest park.
But costs came to more than five times the city would have spent on human landscapers, a report found.
The goats also ate native plants among the invasive species, the report added.
The six-week pilot programme ended last November and there are no plans to renew it, city officials said.
Aside from their cost, the goats left behind a "heavily fertilized area", the city's public works operations manager Mark Becktel told the Statesman Journal.
The park was also said to have a "barnyard aroma" due to the goats' presence.

The new line-up posed inside 10 Downing Street on Tuesday, with the PM flanked by Home Secretary Amber Rudd and Chancellor Philip Hammond.
For some of the details behind the grinning faces, read the BBC's at-a-glance guide to Mrs May's cabinet.
Or take a trip down memory lane below with some of her predecessors' "school photos".

Pacquiao, 37, has not fought since he was convincingly beaten on points by Floyd Mayweather in May.
Promoter Bob Arum says Pacquiao and WBO welterweight champion Bradley, 32, will meet in Las Vegas on 9 April.
Khan also hoped to fight Mayweather in 2015 but the American great retired after beating Andre Berto in September.
Khan, 29, only fought once in 2015, outpointing Chris Algieri in May, and looks set for a clash with Sheffield's IBF welterweight champion Kell Brook in the summer.
"He has a spotty record, hasn't done great ratings, Arum told the Los Angeles Times. "Why would we feel he'd do well on pay-per-view?"
Bradley was awarded a controversial decision against Pacquiao in 2012 before the six-weight world champion earned revenge in 2014.
Arum claims he has agreed deals with both fighters, though the paperwork for the bout has yet to be finalised.
The Bradley fight could be Pacquiao's last, although Arum refused to confirm that would be the case.
The scheduling of a tribunal into the behaviour of a Plaid Cymru AM at the same time as his party's conference gives the appearance of political interference, the party's chief executive has said.
[NEXT_CONCEPT]
Oregon will not be renewing a scheme which uses goats to eat invasive species due to rising costs and complaints over the animals' odour.
[NEXT_CONCEPT]
This is the first official photograph of Prime Minister Theresa May's new cabinet.
[NEXT_CONCEPT]
Britain's Amir Khan has missed out on a fight against Manny Pacquiao, with the Philippine great choosing a third match with American Timothy Bradley instead.