The BBC said it had interviewed 27 former inmates who were held at the US air base between 2002 and 2008 under suspicion of belonging to or helping the Taliban or al-Qaeda.
The former inmates said they were beaten, deprived of sleep, hung from the ceiling and threatened with dogs at the air base outside Kabul.
Allegations of ill-treatment appear repeatedly in the interviews, including the use of stress positions, excessive heat or cold, being forced to undress in front of female guards and, in four cases, being threatened with death at gunpoint, the BBC said.
It said the former inmates had not been given access to legal representation, unlike detainees at the US detention camp of Guantanamo Bay in Cuba.
All were later released without charge.
The 27 had been questioned separately around Afghanistan over a two-month period, and were asked the same questions.
Only two said they had been treated well, the BBC said.
The Pentagon was quoted as denying the charges, and saying all inmates in the facility are treated humanely.
Lieutenant colonel Mark Wright, a spokesman for the US secretary of Defence, told the BBC conditions at Bagram "meet international standards for care and custody".
"Department of Defence policy is and always has been to treat detainees humanely. There have been well documented instances where that policy was not followed, and service members have been held accountable for their actions in those cases," he said.
Reuters