This paper investigates the volume-synchronized probability of informed trading ( VPIN) for the stocks listed in CSI 300 index． Using VPIN as a proxy of information asymmetry，we examine the risk premia of information asymmetry and the relation between information asymmetry and information uncertainty． Our empirical results show that: ( 1) An increases in VPIN results in a larger bid-ask spread and liquidity risk，leading to a pounced risk premia in both cross-sectional and time series analyses． ( 2) VPIN dominates the factor of information uncertainty in explaining the excessive return，while information uncertainty also generates co-directional effects on VPIN． ( 3) VPIN remains significant after we control information uncertainty，size，momentum factors． Meanwhile，Fama-Macbeth analysis shows that the VPIN has no endogenous problem with traditional risk factors and is orthogonal to them． ( 4) Unusual risk premia and trading volumes which occur before companies’major announcements can be captured by the synchronized abnormal VPIN．