Abstract: Aiming at the issue of security analysis on the medium and long-term electricity transactions in China, this paper proposed a step-by-step security analysis method combining monthly elastic pre-checking and day-ahead security analysis. Before the opening of monthly market, the market operator discloses the electricity space of typical day to the market players. According to this, the market operator carried on the elastic pre-check to generator’s market electricity and scheduled electricity. Then, the pre-check results and the risk warnings were formed, and the market players could decide for themselves whether to trade beyond the limit. In the day-ahead market, based on the daily electricity quantity, it a day-ahead check model was established to carry on the security analysis, which aimed to minimize the virtual cost, and considers the latest grid topology and line constraints, load forecasting, clean energy output forecasting, units’ operation constraints, etc. Finally, the effectiveness of the proposed method and model was verified by simulation analysis of the 14-node IEEE system.