Under Mr. Xi, the party’s traditionally suspicious attitude towards business people who are politically active or open-minded has worsened. Wang Gongquan, a former venture capitalist who advocated more liberal social and political policies, was among the first high-profile people to be arrested after Mr. Xi came to power. Ren Zhiqiang, a retired real estate agent, was sentenced to 18 years in prison last year after repeatedly criticizing Mr. Xi’s policies, including the government’s mistreatment in the early days of the coronavirus outbreak.

In private chat rooms and behind closed doors, some people ask what signal Beijing is sending to the private sector by arresting Mr. Sun. Outspoken and generous, Mr. Sun is in some ways the model of the bourgeois businessman the Party extols. He built a city – Dawu City – around his company’s campus in rural Hebei Province, with a 1,000-bed hospital.

“My dream,” he once said, “is to build a modern city in the country.”

Mr. Sun, 66, was born in Xushui, Hebei Province, about a two-hour drive south of Beijing. After graduating from middle school, he joined the People’s Liberation Army. Eight years later, he left the army and returned to his hometown to work at the state Agricultural Bank of China.

A curious and restless soul, he studied law in college and took courses in Chinese literature in his spare time. In 1985 he quit his banking job and opened a shop with 1,000 chickens and 50 pigs. His company, Dawu Agricultural and Animal Husbandry Group, currently employs around 9,000 people, many of them from nearby villages.

As his business grew, Mr. Sun went to Beijing to look for liberal intellectuals. In the spring of 2003, he became a voice for the rights of farmers and entrepreneurs and made speeches at leading Chinese universities.

After irritating the authorities, he was arrested for illegally fundraising. His new friends jumped in his defense. Legal scholars argued that the law he violated was written to give the authorities a wide discretion to indict businessmen who had fallen out of favor.

Liu Xiaobo, the human rights activist who later became a Nobel Peace Prize laureate and died in prison in 2017, then stated that Mr. Sun was “an enormous challenge to the current system.” As an entrepreneur, Mr. Liu wrote, Mr. Sun despised bribery, had the financial resources to act independently, and had the courage to speak out and promote political reform.