Chinese investigators say activist Fu Xiancai, who was paralysed after a severe beating, inflicted the blows himself, according to a rights body.
Mr Fu has helped people displaced by the Three Gorges Dam
Mr Fu, who campaigned for people displaced by the Three Gorges Dam, was beaten up returning home after he was summoned by police in Hubei province.
The June beating was so severe he is not expected to walk again, according to Human Rights in China (HRIC).
But an official investigation has ruled the attack was fabricated.
Officials told Mr Fu's son, Fu Bing, that investigators had failed to find anyone else's footprints at the scene of the attack, and had concluded that he must have hit himself.
The blow to the back of his neck was so severe that three of his vertebrae were broken, HRIC said.
HRIC said it was strongly concerned about the independence of the investigation, which was carried out by the same public security bureau that had a record of harassing Mr Fu.
Mr Fu has highlighted the plight of people moved to make way for the Three Gorges dam.
He had been subject to a series of threats, attacks and harassment in the past year, the group said.
China says the dam, which will be the world's largest hydro-electric project, will provide electricity for its booming economy and help control flooding on the Yangtze River.
But it comes at the expense of villagers, who in many cases have been resettled on inferior land and been deprived of compensation by corrupt local officials, the rights group said.