Mr Nowak - the first UN rapporteur on torture allowed in the country - said abuse was declining in cities, but that "more structural reform" was needed.
Beijing outlawed torture in 1996, but human rights organisations report it is still used to extract confessions.
Mr Nowak visited detention centres in the capital Beijing, and the restive western regions of Tibet and Xinjiang.
Torture methods cited in a statement at the end of his visit included use of electric shock batons, cigarette burns, and submersion in pits of water or sewage.
Mr Nowak spoke of one detainee forced to lie on a bed in a particular position for 85 days.
He also raised concern about psychological torture, particularly in labour camps, which he said was designed to alter the personality of the detainees.
Asked about the prevalence of torture, he answered: "I consider it on the decline, but still widespread."
Mr Nowak said the continuing use of torture was due to pressure on police officers to provide evidence in the form of confessions.
The UN official also accused Chinese officials of systematically interfering with his investigations.
He said the authorities frequently monitored his interviews with relatives of prisoners.
"Victims and family members were intimidated by security personnel during the visit, placed under surveillance, instructed not to meet with him [Mr Nowak] or physically prevented from meeting with him," the statement said.
While interviewing inmates, Mr Nowak said he had observed "a palpable level of fear and self-censorship".
Mr Nowak said that until major legal reforms allowed for an independent judiciary, the problem of torture could not be brought under effective control in China.
"There is much that still needs to be done, there is a need for many more structural reforms," he said.
Mr Nowak's visit came at a time when a public debate is going on in the Chinese media about the use of torture and coercion by the police.
In one recent case, a man who had been sent to prison for murdering his wife was released after she was found alive.
The man, She Xianglin, said he had been tortured into confessing to the murder, and had already served 11 years of his sentence.
A BBC correspondent in Beijing says the fact Mr Nowak was allowed to visit Chinese prisons - after a decade of failed attempts - does indicate the country's leaders are willing at the very least to acknowledge the problem.