The increase of direct power purchase may reduce the regulation capability to peak /valley loads and intensify the loading of transmission sections，which yield more diverse utilization hours of the thermal units with different capacities，thus add difficulty to power dispatching． Therefore，it is necessary to decide the upper limit of direct power purchase and analyze the influencing factors． The quantitative calculation method for the upper limit of direct power purchase is proposed． Based on the historical load data，the load is forecasted． The upper limit of direct power purchase can be quantified by adjusting the thermal units with large or small capacities． The relative factors，such as the regulation capability to the peak /valley loads， transfer capability of critical transmission sections and the reduction coefficients are analyzed to derive the evaluation indices． The numerical results of a practical system verify the feasibility and accuracy of the proposed algorithm and indices．