Nowadays power dispatching center needs to check the trading energy of power plant in monthly market to ensure that the trading energy meets the operation requirements of the actual situation. To solve this problem, a two-stage checking method is proposed in this paper. The first stage is monthly power distribution based on a monthly unit commitment model to confirm the schedule plan for the unit and distribute the monthly power into each day. The second stage is typical day operation simulation based on a daily economic dispatch model to check whether the generation curve of each plant can meet the requirements of power grid security. Based on two-layers optimization theory, the two-stage problem mentioned is iterated repeatedly and solved. This method can check whether the trading result meets the operation requirements and provide a useful method about the maximum trading power of a certain power plant. The validity of the proposed method is verified by a case study in IEEE 30 buses system.