Abstract: To address the problem that the calculation accuracy and speed of MMC losses are difficult to balance, a fast online evaluation method of losses is proposed. First, based on the determined modulation and capacitor voltage balance strategy, the switching frequency surface is constructed to reflect the switching frequency under any steady-state working condition. Next, the upper limit of switching frequency and the upper limit of switching losses are calculated by using the average value model. Once again, combined with the calculation results and the switching frequency surface, the rapid calculation of the actual losses is realized with high accuracy. Then, considering the impact of losses on the simulation results, the energy absorbed by a controlled voltage source connected in series on the bridge arm is used to characterize the losses. A switching loss injection method based on attenuation function is proposed to avoid injection voltage spikes and improve injection efficiency. Finally, an MMC average simulation model considering loss injection is built in PSCAD to verify the computational efficiency and accuracy of the proposed method.