Clara Buckley, whose son Orville Blackwood died in August 1991, said parts of the report she had been allowed to see were misleading.
Last night, she accused the Special Hospitals Service Authority, which runs Broadmoor, of exaggerating the problems posed by her son in order to provide an excuse for his death.
In April, an inquest jury returned a verdict of accidental death on Mr Blackwood, a schizophrenic who had been admitted to Broadmoor in 1987 after trying to rob a post office with a toy gun. The jury was told he was forcibly injected with 150mg of the drugs Sparine and Modecate after he hit a doctor. He died of heart failure.
Mr Blackwood was the third black man to die in similar circumstances at Broadmoor, prompting the authority to launch an investigation. The report, due out tomorrow, is expected to consider the use of drugs and solitary confinement at all special hospitals.
But Mrs Buckley has been shown parts of the report relating to her son's past, and she is deeply unhappy. She says it depicts his childhood as beset by arguments. 'But we had no more arguments than every family.'
He fell foul of authority on a number of occasions, but Mrs Buckley says many of the incidents have been sensationalised. For instance, in 1982, he was taken to hospital by four police officers in what the authority paints as a dramatic confrontation, she said. In fact, he went peacefully, she claimed.
If her son was 'restless, aggressive and disruptive', as the report suggests, it was only because Broadmoor staff did not know how to handle him.
Mrs Buckley, who lives in south London, persuaded the High Court to quash the verdict of a first inquest in October 1991 because the coroner refused to let the jury consider whether lack of care contributed to her son's death. However, she was unhappy that the second inquest came to the same conclusion as the first - that he died as the result of an accident.
The authority was unavailable for comment yesterday.