Mr Cheng, 29, told the BBC he was “shackled, blindfolded, and hooded” and claims he was beaten and forced to sign confessions.
He was accused of inciting political unrest in Hong Kong.
They wanted to know what role the UK had in the Hong Kong protests - they asked what support, money, and equipment we were giving to the protesters
Mr Cheng said his captors “said they work for the secret service and that there are no human rights.
“They would beat the bony parts, like my ankles… or any vulnerable part.”
Mr Cheng said the Chinese authorities accused Britain of supporting demonstrations in Hong Kong.
He recalled: “They said I’m a state enemy and I’m a traitor, and also they asked whether the consulate instructed me to mingle with the protest.”
He added: “They wanted to know what role the UK had in the Hong Kong protests – they asked what support, money, and equipment we were giving to the protesters.
“I told them I want to make it 100% clear, the UK didn’t assign resources or help with the protests.”
Unrest in the city state began in the summer with calls to scrap a now-suspended extradition bill, but have now widened to protest against the way Hong Kong is administered by Beijing.
Dominic Raab has said the Government was “shocked and appalled” by Mr Cheng’s treatment which he believes “amounts to torture”.
He said: “Simon Cheng was a valued member of our team.
“I summoned the Chinese Ambassador to express our outrage at the brutal and disgraceful treatment of Simon, in violation of China’s international obligations.
“I have made clear we expect the Chinese authorities to investigate and hold those responsible to account.”