As the continuous advance of electricity market reform, medium and long-term electric power contracts play an important role in risk management, market stability and other aspects, so it is quite necessary to study medium and long-term electric power contracts. This paper analyzes the transaction process and mathematical model of the medium and long-term electric power contracts under different organizational modes. Finally, the decomposition of contract electricity quantity is analyzed. The bidirectional correction method is adopted in this paper, and the method is refined and perfected. Through this method, the contract electricity quantity can be decomposed rapidly in various practical situations.