Medium and long-term trading power is increasing, and the adjustable margin of grid is reduced. Therefore, medium and long-term electric quantity security check is necessary. Considering power quantity constraints and the future state grid model, the power quantity margin solution model is constructed. The power quantity limits of principal equipments such as unit group and line section are calculated, and the medium and long-term electric quantity security check and adjustment are realized through the correlation coefficient matrix. The provincial power grid is used as an example to verify the effectiveness of the proposed model and method. The method of power calculation and electric quantity check provides a technical analysis method for medium and long-term bilateral trading and grid security dispatch operation. This work is supported by Science and Technology Project of State Grid Corporation of China (No. 5442DZ180005) “Research and Application of Key Technologies for Medium and Long-Term Electric Quantity Security Check”.