Junead Khan, 25, of Marlow Avenue in Luton, downloaded a bomb-making recipe and browsed the internet for a knife used by British militant "Jihadi John".
He was jailed for life in May for preparing a terror act.
This has now been replaced with a 20-year jail term with five years on licence.
The Appeal Court heard Khan opened the bomb pack computer file "only once" and, although he looked for a knife, he never placed an order.
Although he went close to US military bases in East Anglia, including RAF Mildenhall in Suffolk, he never deviated from his planned route.
"There is no doubt about the heinous nature of the crime, which Junead Khan formed an intention to commit," the judge said.
"In terms of steps actually taken, he did not carry his intention far."
He had committed a serious crime but not one that needed a life sentence, the panel of judges including Lord Justice Treacy and Mr Justice Leggatt, concluded.
The judges also dismissed an appeal by Khan and his uncle Shazib Khan, 24, also of Marlow Avenue in Luton, against their sentences of seven-years jail, with five years of extended licence, for preparing terrorism offences.
The pair had planned throughout 2014 and 2015 to travel to Syria to live under the so-called Islamic State regime, but failed to raise enough money for the fare and to buy all the kit their IS contacts had asked them to bring.

Bassel Khartabil's wife said he was killed in October 2015 after receiving a death sentence.
"Words are difficult to come by," she wrote. "Thank you for killing my lover."
The Wikipedia contributor and founder of Creative Commons Syria was first detained in March 2012.
After nine months of incommunicado detention, he was moved to Adra prison in Damascus, where his family were allowed to visit him.
But he was moved from the prison to an unknown destination in October 2015, and supporters had until now held out hope he might be alive.
Soon after his disappearance in 2015, his wife Noura Ghazi Safadi said she did not know if he was alive or dead.
But she said she had been contacted by self-declared Syrian government insiders who told her that he had been sentenced to death.
Bassel Khartabil, a Syrian-Palestinian, was named a Top Global Thinker by Foreign Policy magazine in 2012 and given the Digital Freedom Award by Index on Censorship in 2013.
In 2013, he was described by two European parliamentarians as being "credited with opening up the internet in Syria - a country with a notorious record of online censorship - and vastly extending online access and knowledge to the Syrian people".
Noura Ghazi Safadi described her husband as "unbelievably romantic" in a 2015 interview with the Electronic Intifada news website, adding that he "wasn't a typical internet geek".
"He used to give me a red rose every day since we became lovers until the day of his imprisonment," she said.
Khartabil, an open source web developer, had co-founded the #NewPalmyra project, which creates digitally reconstructions of the ancient Syrian city.
Wikipedia founder Jimmy Wales said on Twitter that he was "saddened and outraged in equal measure" to learn of the execution.
A delivery driver jailed for life for plotting to kill US personnel outside an air base has had a new sentence imposed by the Appeal Court.
[NEXT_CONCEPT]
A Syrian internet freedom activist was executed soon after he disappeared from prison in late 2015, his wife and supporters say they have learned.