The Washington Post reported Thursday that in the previously secret transcript, Zhao Ziyang, who died four years ago, said the government's decision to impose martial law around Beijing in May 1989 was illegal.
He also wrote that a peaceful end to the student-led democratic protests in Tiananmen Square could have been negotiated.
The memoir is due to be released next week.
Zhao's memoir was dictated onto audiotapes while he was under virtual house arrest. The publisher has titled his work "Prisoner of the State: The Secret Journal of Premier Zhao Ziyang," the newspaper reported.
In his memoir, Zhao takes aim at, among others, Deng Xiaoping, who is credited with opening China to the West; Li Peng, the dour premier at the time of the Tiananmen massacre; and Deng Liqun, a hard-line party theoretician, the Post reported.
"Reading Zhao's unadorned and unboastful account of his stewardship, it becomes apparent that it was he rather than Deng who was the actual architect of reform," Roderick MacFarquhar, a professor of Chinese history at Harvard University, wrote in a foreword to the book.