The invention relates to a hydropower station group monthly transaction plan electric quantity decomposition method considering power grid section constraints, and belongs to the technical field of hydropower dispatching operation. According to the method, the monthly transaction plan electric quantity decomposition scheme of the hydropower station group can be quickly determined by adopting the multi-dimensional search algorithm by taking minimum water consumption in the scheduling period as a target and considering the main transmission section control limit of the power grid. Firstly, monthly transaction plan electric quantity of each power station is extracted, and a hydropower station group water consumption minimum model considering power grid transmission section limitation is constructed; secondly, a multi-dimensional search algorithm is constructed from three dimensions of time, a power station and a feasible region, a check correction strategy is introduced for power grid section limit constraints, namely, loosening is performed, and then optimization calculation is performed, whether the output of each section violates the constraints one by one according to an optimization result is checked, and if the output violates the constraints, an equal proportion reduction and load transfer strategy are adopted to ensure that section safety constraints are met. The power generation water consumption of the system can be effectively reduced, and a technical thought is provided for medium-and-long-term transaction plan execution in a power market environment.