The New York-based Human Rights in China said on Wednesday Sang Jiancheng was at least the sixth person imprisoned for signing and distributing an open letter released before a major Communist Party meeting in November 2002.
Sang was sentenced on Tuesday by the Shanghai Intermediate People's Court, the group said.
Sang's other postings appealed for help for the unemployed and people who have lost their homes to redevelopment projects, according to human rights groups.
He reportedly accused the party of corruption - a sensitive accusation amid widespread public disgust over official graft and abuse of power.
The official response to the letter highlights the party's hard line against political dissent, despite reforms meant to make government more responsive to the public.
Other signers of the open letter, most of them veteran activists, have been sentenced to up to eight years in prison on subversion charges.
Meanwhile, Beijing police arrested two people for plotting a protest in Tiananmen Square by 500 people with complaints against the government, state media said on Wednesday.
Police arrested Sun Shuping and Wu Daming for gathering the crowd to head to Tiananmen Square, China's political epicentre, on 12 December, the Beijing News reported. The protesters wore matching clothes and carried flyers, it said.
"Tiananmen Square is not a government department, especially not the department for accepting complaints or letters of complaint. It is a window that represents the image of the nation and the Chinese people"
Beijing police chief
"The incident seriously disturbed order and public safety in the Tiananmen area," the newspaper said.
The protesters had various complaints, said another newspaper, the Beijing Morning Post without elaborating.
The crowd was stopped before entering the square, where thousands of student-led pro-democracy demonstrators camped out for months in 1989.
Those protests were eventually crushed by the army in June that year.
Protests are common in China and all levels of government have offices where people can air complaints. Among the issues have been land disputes and unpaid wages and benefits by workers from failing state-owned enterprises.
Sun and Wu had applied for permission for the protest, but it had been denied, the Beijing News said.
"Tiananmen Square is not a government department, especially not the department for accepting complaints or letters of complaint," police chief Ma Zhenchuan was quoted as saying.
"It is a window that represents the image of the nation and the Chinese people... Anyone who disturbs the normal order in Tiananmen Square will be punished."