Mr Humphrey and his wife, Yu Yingzeng, were sentenced in August last year for illegally obtaining private information on Chinese nationals.
He received a two-and-a-half year sentence, while Ms Yu was sentenced to two years in prison.
GlaxoSmithKline (GSK) China, which was fined $490m in China for bribery, hired Mr Humphrey in 2013.
The pair were hired by GSK to investigate the source of a sex tape of the company's China boss, shortly before the firm became the subject of the bribery allegations.
They admitted buying information, but said they did not realise it was illegal to do so.
Release in the coming days will reduce his sentence by seven months. Time spent in custody before his trial was taken off his sentence.
Once released, he is likely to be deported, as the court originally stipulated.
Ms Yu will be released in the coming weeks, according to the Reuters news agency.
The original verdict did not make a link between Mr Humphrey's investigations and the allegations against the pharmaceutical giant GSK.
Those allegations first emerged in an email in January 2013 from an anonymous and self-styled whistleblower to the company.
The email alleged that GSK's sales teams targeted influential doctors with expensive gifts and cash to win business.
It was sent to GSK's London-based chief executive, Andrew Witty, and made accusations against GSK China's general manager Mark Reilly.
Later, a sex tape featuring Mr Reilly and his girlfriend was also sent.
GSK declined to comment.

Gwen Smith, 64, said she went up to Mary Logie's flat in Leven, Fife, after hearing the noises.
She told a court that the woman accused of the murder, Sandra Weir, had answered the door of Ms Logie's flat.
Ms Weir denies murder and has lodged a special defence of alibi claiming she was elsewhere in Leven at the time.
Ms Smith told the High Court in Edinburgh that she lived directly downstairs from the pensioner and went upstairs to see what was going on after hearing "hammering" noises.
On the third day of the trial, Ms Smith said she had then come across a "horrible situation."
She told prosecution lawyer Alex Prentice QC: "Sandra says to me 'she's fell and there's blood everywhere.'
"I said 'it's alright calm down I will go and see' and then I opened the door and there's Mary."
Ms Smith said that Ms Logie was lying on the floor of her living room.
When Mr Prentice asked her if she saw an item lying nearby, Ms Smith replied: "A rolling pin on the couch."
Ms Weir denies eight charges, including murder.
The trial also heard from a man who told the court he had sold Ms Weir drugs on a number of occasions.
Before giving evidence, Mr Prentice told Aaron Robertson, 44, that if any of his answers implicated him, he guaranteed him immunity from prosecution.
Mr Robertson said Ms Weir was visually impaired and would come to his home with a guide dog to buy drugs.
He said he gave her heroin four or five times in January, charging Â£70 for a 16th of an ounce.
He said Ms Weir had built up a debt of Â£320, which was paid off on 14 January.
The trial at the High Court in Edinburgh continues.
British company investigator Peter Humphrey is to be released early from a Chinese jail, according to reports.
[NEXT_CONCEPT]
The neighbour of an 82-year-old woman allegedly beaten to death with a rolling pin has told how she heard "loud bangs" coming from the flat.