“It’s very stressful in China — sometimes I was working 128 hours a week for my auditing company,” Ms. Chen said in her Beijing apartment a few hours before leaving. “And it will be easier raising my children as Christians abroad. It is more free in Australia.”

At 30, Chen Kuo had what many Chinese dream of: her own apartment and a well-paying job at a multinational corporation. But in mid-October, Ms. Chen boarded a midnight flight for Australia to begin a new life with no sure prospects.

Like hundreds of thousands of Chinese who leave each year, she was driven by an overriding sense that she could do better outside China. Despite China’s tremendous economic successes in recent years, she was lured by Australia’s healthier environment, robust social services and the freedom to start a family in a country that guarantees religious freedoms.

“It’s very stressful in China — sometimes I was working 128 hours a week for my auditing company,” Ms. Chen said in her Beijing apartment a few hours before leaving. “And it will be easier raising my children as Christians abroad. It is more free in Australia.”

As China’s Communist Party prepares a momentous leadership change in early November, it is losing skilled professionals like Ms. Chen in record numbers. In 2010, the last year for which complete statistics are available, 508,000 Chinese left for the 34 developed countries that make up the Organization for Economic Cooperation and Development. That is a 45 percent increase over 2000.

Individual countries report the trend continuing. In 2011, the United States received 87,000 permanent residents from China, up from 70,000 the year before. Chinese immigrants are driving real estate booms in places as varied as Midtown Manhattan, where some enterprising agents are learning Mandarin, to the Mediterranean island of Cyprus, which offers a route to a European Union passport.

Few emigrants from China cite politics, but it underlies many of their concerns. They talk about a development-at-all-costs strategy that has ruined the environment, as well as a deteriorating social and moral fabric that makes China feel like a chillier place than when they were growing up. Over all, there is a sense that despite all the gains in recent decades, China’s political and social trajectory is still highly uncertain.

“People who are middle class in China don’t feel secure for their future and especially for their children’s future,” said Cao Cong, an associate professor at the University of Nottingham who has studied Chinese migration. “They don’t think the political situation is stable.”

Most migrants seem to see a foreign passport as insurance against the worst-case scenario rather than as a complete abandonment of China.

A manager based in Shanghai at an engineering company, who asked not to be named, said he invested earlier this year in a New York City real estate project in hopes of eventually securing a green card. A sharp-tongued blogger on current events as well, he said he has been visited by local public security officials, hastening his desire for a United States passport.

“A green card is a feeling of safety,” the manager said. “The system here isn’t stable and you don’t know what’s going to happen next. I want to see how things turn out here over the next few years.”

Political turmoil has reinforced this feeling. Since early this year, the country has been shocked by revelations that Bo Xilai, one of the Communist Party’s most senior leaders, ran a fief that by official accounts engaged in murder, torture and corruption.

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