In order to perform contract settlement， generation companies' mid-long-term energy contract for difference need to be decomposed under electricity market operation. Under the traditional contract decomposition approach， the contracts are decomposed in a centralized way and generation companies accept the result passively. A new contract decomposition method named generation company declaration method is presented. This method considers maintenance， generating unit capacity， and other factors. A difference evaluation function based on errors is adopted to build a multiple objective optimization model. The linear weighted sum method and minimax method are respectively selected to convert the original problem into a single objective optimization. A case study of a domestic market is performed to indicate that the suggested model is effective and the solution of the original problem can be obtained by both conversion methods. It has been observed that when minimax method is selected， the difference in generation companies' adjustment quantity is smaller.