Syrian national Amin Al-Sheikh, who admitted fighting US forces in Iraq, claimed he was beaten by Graner while recovering from bullet wounds.
He alleged the army reservist threatened to kill him more than once, and laughed, whistled and sung while he assaulted him.
In videotape evidence to Graner’s court martial at an army base in Foot Hood, Texas, Al-Sheikh said the 36-year-old made him eat pork and drink alcohol, in violation of his vows as a Muslim.
“Graner is a man who represents the people that are sick-minded,” he said.
He also said he listened through his cell wall while Graner and other Americans forced another prisoner to eat from a toilet.
Charged with assault, conspiracy to mistreat prisoners, dereliction of duty and committing indecent acts, he faces a maximum sentence of up to 17 years in a military prison.
His defence is primarily that he was obeying orders, while prosecutors allege Graner was the ringleader of abuse at Abu Ghraib
Al-Sheikh’s evidence appeared to boost the prosecution case, with the former prisoner claiming Graner and other military policemen, rather than interrogators from military intelligence, were the ones who conducted most of the beatings.
“Graner and those torturers were the ones doing the torture. They were the experts,” he said.
Al-Sheikh was wounded in the chest and legs in a gun battle with US soldiers in October 2003.
He said while in his cell, Graner jumped on his wounded leg and hit his wounds with a metal baton.
Asked if it hurt, he said: “You cannot imagine. I cried.”
He said one of his legs did not heal properly. “It’s not straight, it’s bent,” Sheikh said.
Sheikh, who alternated between speaking in English and Arabic, said a Yemeni detainee told him how Graner had made him “eat from the toilet.”
On another occasion, he said, Graner handcuffed him to his cell door with his arms behind his back for eight hours.
Graner and Private Lynndie England, with whom he fathered a child and who is also facing a court-martial, became the faces of the Abu Ghraib scandal after they appeared smiling in photographs that showed degraded, naked prisoners.
The pictures further eroded the credibility of the United States already damaged in many countries by the Iraq invasion.
At the trial military prosecutors have presented evidence not seen before in public from Abu Ghraib, including a video of forced group masturbation and a picture of a woman prisoner ordered to show her breasts.
On Monday witnesses described how Graner forced prisoners to take off their clothes and masturbate, and piled naked detainees on top of each other.
Graner is the first soldier charged in the Abu Ghraib scandal to contest the charges against him.
Four others received sentences ranging from demotion to eight years in prison.