Abstract: Electric vehicles（EVs） can balance grid voltage through V2 G（vehicle-to-grid） technology as mobile storage, but their temporal-spatial dual uncertainty will affect the safe operation of the grid. To solve this problem, considering the uncertainty of EVs, this paper proposes an evaluation method for V2 G response-ability and energy storage capacity of EVs. The temporal-spatial behavior of the EVs is simulated by Monte Carlo method using the random travel chain. On the basis of the Gaussian mixture model, the probability model of the charging station load and the SOC（state of charge） of the EVs in the station are established. The voltage regulation ability index and the V2 G response capacity index of EVs are then proposed to assess the response-ability of EVs in different periods. The simulation compares the dispatchable capacity of EVs and the recovery effect on grid voltage in different periods. Finally, the results verify the effectiveness of the proposed evaluation method.