According to China Aid Zhu YuFu, 65, was confronted by a police officer about the length of his beard on 11 February.
Zhu attempted to explain that his razor had been locked away before the officer attacked him, causing him to hit his head on concrete and lose consciousness.
Zhu had to be given emergency medical treatment for his injuries.
He already suffers with heart disease and a number of other blood and vascular ailments.
His wife Jiang Hangli was prevented from visiting him in the prison in Zhejiang province that morning and found out the details about his ordeal after she was eventually allowed to see him later that day.
Zhu was a founder of the Democracy Wall movement, a political group that campaigned against government corruption and human rights abuses. He also published a monthly magazine about politics.
He was first convicted of trying to subvert state power in 1999 and served a seven-year prison sentence.
He was arrested again shortly after his release and charged with obstructing government business.
Zhu became a Christian in 2010. He is currently serving a seven year prison sentence.
Because of his declining health, his family fear that Zhu may die in prison but all requests for medical parole have been denied.