In order to improve the practicability and economy of vehicle-to-grid (V2G) accessing power system, the paper proposes a novel unit combination algorithm considering plug-in electric vehicle (PEV) quantity change and state-of-charge (SOC) change. The relationship between SOC and charge and discharge power along with the estimate of PEV quantity in different time is used to determine daily PEV net charge demand. A new unit commitment in power system considering V2G model is developed, taking the generating cost as the objective function and the state-of-charge as the penalty function to figure out the output schedule and the optimal PEV charging and discharging control plan. The optimal PEV charging and discharging and the results of unit combination in different scenarios are investigated. The effects of different PEV charging modes on the unit combination are compared. It is found that the method can save the unit cost effectively and realize the continuous dispatching, and its correctness and applicability are verified.