the Basic Healthcare Insurance for Urban Employees (BHI) in China has accumulated 406 billion surpluses in 2009.Why does this PAYG designed social health insurance (SHI) program have so much balance? This paper examines the causes of the high level of balance,using a provincial level panel database ranging from 1999-2008.Statistics results indicate that huge surplus is related to low dependency ratio.the increase of wage and BHI contributions.the steady growth of BHI coverage.Also, BHI surplus is produced by low level of BHI expenditure in reimbursement and government financial capacity.Besides, the medical savings accounts (MSAs) and level of risk pooling result in a significant share of BHI surplus.Although being natural causes, demographic structure, income growth and extension of BHI coverage need to be watched over closely, because they are transitory rather than permanent causes.In order to dissolve these unnatural BHI surplus, China needs to spend more BHI funds in paying patients health expenditure, raise the risk pooling level, or adopt flexible contribution rates.The findings are useful to SHI researchers and administrators from other countries, because level of balance is closely related to SHIs governance, attractiveness and sustainability.