Last year's outflow was part of a $3.8 trillion flood of capital that left China over 11 years, Global Financial Integrity said. It said the amount rose from $172.6 billion in 2000 to $602.9 billion in 2011.
The group said it was unclear how much of the money came from corruption or other crimes but it said the illicit outflow could aggravate economic and political strains by aiding tax evasion and widening China's sensitive wealth gap.
The study "raises serious questions about the stability of the Chinese economy," the group said in a statement.
"The social, political, and economic order is not sustainable in the long-run given such massive illicit outflows," said Dev Kar, the group's chief economist and a co-author of the report, in the statement.
Global Financial Integrity, a program of the Center for International Policy, studies illegal cross-border flows of money and promotes measures to stop them.
Some commentators have suggested the outflows reflect a loss of faith by China's financial elite in the communist government but others say much of the money is sent back into the country disguised as foreign investment.
"While the funds could be earned through bribery, kickbacks, or other illicit activities, they may well be earned through legitimate means," said Global Financial Integrity. "It is the transfer in contravention of capital controls or the nonpayment of applicable taxes that renders the funds illicit."
Some of the outflow also might be the proceeds of bribe-taking and other government corruption. Stories of officials who move their families and ill-gotten assets abroad are so common that Chinese Internet users have coined a name for them "naked officials."
Public concern about chronic corruption and China's slowing economy are political risks for the ruling Communist Party as it prepares for a once-a-decade handover of power to younger leaders.
A survey released last week by the Pew Global Attitudes Project found half of Chinese people who responded cited corrupt officials as a major problem, up from 39 percent four years ago. Forty-eight percent cited the wealth gap as a major problem, up from 41 percent in 2008.
Global Financial Integrity said 86 percent of the money moved abroad, or $3.2 trillion, was moved out of China through "trade misinvoicing," under which Chinese companies arrange with foreign suppliers to overcharge for imported goods and deposit the extra money abroad.
Chinese economists have said a large share of the country's foreign direct investment from financial centers such as the Cayman Islands, Luxembourg or Hong Kong is money that was first sent abroad by Chinese companies.
Global Financial Integrity said that one financial center, the British Virgin Islands in the Caribbean, has just 28,000 people but accounted for $213.7 billion in officially reported investment in China in 2010.
"Clearly, genuine recorded FDI into China is overstated to the extent that total foreign direct investment includes round-tripped funds coming from tax havens," said Kar.
The financial flows reflect widespread frustration with China's government-controlled financial system, which channels most bank lending and other benefits to state-owned companies at the expense of savers and entrepreneurs.
The government has eased some controls by allowing families to move more money abroad to buy real estate or make other investments and reducing the number of approvals required for companies to invest abroad.