Mam Sonando, owner of an independent radio station, was convicted of "insurrection" and sentenced to 20 years in prison in October.
He allegedly incited villagers to form their own state in the eastern province of Kratie.
The Appeal Court dismissed some of the charges and gave Sonando a suspended five-year sentence instead on lesser charges, meaning he is likely to walk free soon, a report by the BBC said.
The court's change of heart comes as the country heads into parliamentary election in July.
Amnesty International, which welcomed Sonando's reduced sentence, said it considers him a prisoner of conscience "imprisoned for peacefully exercising his right to freedom of expression."
Rights groups said clashes between government troops and villagers last year happened as they protested what they considered illegal land seizures by big corporations.
A 14-year-old local girl was killed by security forces during the confrontations.
A smiling Sonando in a blue dress shirt and tie and accompanied by two guards arrived at the Appeal Court in the capital, giving two V for victory signs.
The five-year prison sentence was for offenses including "instigating illegal clearing and occupation of forest" and he likely will be released this week, Amnesty International said in a statement.
"We welcome Mam Sonando's imminent release, which is a step in the right direction for freedom of expression in Cambodia," Rupert Abbott, Amnesty International's researcher on Cambodia who attended the daylong hearing, said.
"But Mam Sonando should never have been imprisoned in the first place and the convictions that stand appear baseless. The introduction of new charges at such a late stage raises concerns about the fairness of the proceedings, while the suspended sentence may be designed to silence Mam Sonando."
Abbott said he hoped the appeal court's decision signals a shift in the government's attitude toward dissent as the country heads into national parliamentary elections in July.
However, this week the independent election monitoring group Committee for Free and Fair Elections said it feared the July elections won't be free and fair.
Cambodia's democracy is "increasingly fragile" and the electoral process is excluding opposition and dissenting voices, a report by Comfrel said.
With the country under dominant control of Prime Minister Hun Sen's ruling Cambodian People's Party, the political process "showed trends toward authoritarianism."
The 55-page report "Democracy, Elections and Reform in Cambodia 2012" released this month by Comfrel said "the political pattern to limit political participation, freedom of expression and assembly and access to media was observed in 2011 and continued in 2012."
Comfrel was set up by the U.N. Transitional Authority in Cambodia to monitor the 1993 elections and was recognized as a permanent monitoring group by the Cambodian government in 1995.
"This includes a lack of an open and constructive political discourse on issues of public concern, a continued weakening of the multiparty system and continuing limitations on political participation by the Cambodian public."
Comfrel Director Koul Panha said the CPP dominates court officials, the armed forces and the police, putting them under the party's control.
"I am worried about this political trend that is leading the country to a fragile democratic process. Cambodia is heading toward a one-party state," he told Radio Free Asia.
"This means that military, police and judicial officers will not be impartial during the elections."
But a spokesman for Cambodian government's Council of Ministers refuted Comfrel's report, calling it biased against the government to please Comfrel's foreign donor agencies.
"This (organization's annual) report has a tendency of pressuring the government. Comfrel exists through its donors but the government is elected," council spokesman Phay Siphan was quoted as saying by RFA.
Siphan said there were reports including a 2011 opinion poll conducted by the U.S. government-funded International Republican Institute that showed most Cambodians believe the country is moving in the right direction.