With the continuous increase in the proportion of power electronic sources, the inertia level of the power system continues to decrease, the anti-interference ability continues to weaken, and frequency security is facing severe risks. This paper proposes a method for evaluating the maximum infeed power of power electronic sources considering inertia and frequency security constraints. Firstly, an optimization and simulation model was constructed. Increase the inertia based on the difference between the frequency extreme deviation and its constraint value to optimize unit commitment, and build a frequency response model for the entire unit corresponding to the unit commitment. Then simulate to obtain frequency security indicators for verification. Based on the dichotomy, update the upper and lower limits of the dichotomy according to the frequency security indicators verification results, and iteratively search for the maximum power electronic sources infeed power considering frequency security constraints. Finally, the effectiveness of the proposed method was validated based on an actual regional power grid.