His lawyers, Chen Jiangang and Zhao Yonglin, said in June that prosecutors had threatened to kill Yang and his family if he did not confess.
The lawyers are now suing for the use of "torture to extort a confession".
During a recent visit to Yang in prison, Chen and Zhao found that he is suffering from serious health conditions, they told China Aid.
The pastor is reportedly suffering from liver pain "along with various other serious diseases".
No date has yet been set for his trial.
In a letter sent to his wife Wang Hongwu in June, Yang urged her to hold on to faith and trust God.
"You shouldn't be too anxious. It is best to remain peaceful and wait for God," he said. "God is omniscient. I believe that he never makes a mistake... Let's pray and leave other things to God. Thank you for investing everything in this family. I am grateful for what God has given me; let's carry the cross and hasten down the road to heaven, until the day we meet the Lord."
He requested a recent photo of their sons and said he had "adjusted" to life in detention.
"As the Book of Isaiah says, 'In repentance and rest is your salvation, in quietness and trust is your strength' [Isaiah 30:15]. It's also a kind of rest, staying inside," Yang said.
He ended the letter by encouraging Wang to "always be happy and peaceful. Never stop praying. Be grateful, because that's God's will conveyed through Jesus Christ."
The ruling Chinese Communist Party is believed by human rights campaigners to be becoming increasingly concerned about the influence of Christianity in the country.
A report published by China Aid earlier this year found that persecution against Christians in the country has increased seven-fold since 2008.