Pastor Zhang Shaojie from Nanle Country Christian Church has been fed meagre food rations and deprived of sleep, according to his daughter who is based in the US.
Esther Zhang Huixin told China Aid: "They cruelly torture my father. He's unable to see the sun during the day. He's deprived of sleep for 24 hours at a time. The prison gives him only one steamed bun a day and intentionally starves him.
"According to people who have been released from that prison, my father is barely alive, suffering both mentally and physically."
Pastor Zhang was sentenced to twelve years in prison in 2014 and fined 100,000 Chinese Yuan (£11,500) for 'fraud' and 'gathering a crowd to disturb public order'.
Christian Solidarity Worldwide's Chief Executive Mervyn Thomas said, "Recent reports that he [Pastor Zhang] is on the verge of death after being tortured in prison would be an unacceptable abuse of human rights even if he were guilty, but the deliberate ill-treatment of an unjustly sentenced pastor is an utterly grievous injustice.
His detention came after he, and several members of his church - which is registered under the state-sanctioned Three Self Patriotic Movement (TSPM) - petitioned authorities over a land dispute involving the church.
Mervyn Thomas went on to say: "We call on the Chinese authorities to immediately stop the mistreatment of Pastor Zhang and to review his sentence with a view to securing his unconditional release.
"We further urge the Chinese government to ensure that the right to freedom of religion or belief is fully protected in China."
Some church members fear Pastor Zhang was deliberately targeted as part of a plan by authorities to replace him as leader of the local TSPM group, as punishment for his defence of marginalised individuals.