Yu, a journalist and critic for a Hunan newspaper, was one of three men arrested for throwing paint at the Mao portrait in May 1989, at the height of the Tiananmen Square pro-democracy protests.
The three were later jailed for "counter-revolutionary destruction and counter-revolutionary incitement".
One of the three, Lu Decheng, was released in 1998, and later visited Yu in prison.
Lu told Radio Free Asia in 2004 that his friend was "barely recognisable".
"He had a big scar on the right side of his head. A fellow prisoner said Yu had been tied to a electricity pole and left out in the hot sun for several days. He was also kept in solitary confinement for two years and that was what broke him," Lu said.
Feb 23, 2006
Tiananmen protester leaves prison a damaged man
Yu Dongyue was released from prison after 17 years, he does not recognize his family and cannot speak intelligibly.