Zhai is among the 300 activists and lawyers arrested last year as part of President Xi Jinping's crackdown on legal activism. Twenty individuals are still detained.
The Chinese authorities said that the 55-year-old activist "had long been influenced by anti-China forces and gradually adopted ideology to overthrow the current state system."
Government-run media Xinhua said that the activist was "subverting state power" by organizing sit-ins and demonstrations outside courthouses.
He and the other detained activists are associated with the Fengrui Law Firm in Beijing. Other lawyers who are facing similar charges are Hu Shigen, Zhou Shifeng, and Li Heping.
Wang Yu, another prominent lawyer from the law firm, was released on bail on Monday.
Yu was reported to be free after swearing that she will not go back to practicing law. Her whereabouts are still unknown.
In a video presented to media, Yu also said that Zhou Shifeng as "unqualified" as a lawyer and that "foreign forces" were using the law firm to discredit the government.
The lawyer said that she was "ashamed and remorseful" for her actions, according to the Global Times, a tabloid run by the Communist Party.
Activists claim that the video was taken with coercion.
Families of the detained activists said that the charges were "ridiculous and evil." They were not allowed to attend proceedings.
Zhai's wife has disappeared. Other family members fear that they will also be "taken away."
Wives of the activists went to protest at the Supreme People's Procuratorate, the top prosecutor's office, in Beijing last month. They were demanding that they see their husbands in jail.
"For one year, we've heard nothing about our husbands except an arrest notice. I'm worried about his health, I wish he could get back home soon," said Li Wenzu, wife of Wang Quanzhang,
China has been internationally criticized for disrespecting human rights. The Chinese government maintained that they are only practicing the rule of law.