Primary frequency regulation plays a significant role in power system frequency control. As a result of the separation of plant and grid, it becomes necessary for the grid to monitor the primary frequency performance of generators. This paper designs an online evaluation system of primary frequency regulation based on WAMS. Firstly, the evaluation indices system is proposed, which assesses the primary frequency performance of units from perspectives of static performance, dynamic performance and the regulating process. And the responding algorithms are designed for each index. In this part, a method of Variance Comparison, which helps to find the frequency changing point accurately, is presented to solve the key problem in calculation process. Besides, the impact of electromechanical transient process is taken into account when calculating the contribution electric quantity index and a practical modified method is presented. Computer simulations based on the large frequency disturbance in Hubei Grid are conducted, and the results illustrate the feasibility and efficiency of the proposed algorithms.