Compared with Two-Level circuit, Three-Level circuit has higher chopper frequency and lower harmonic, which is beneficial to the optimal design of the filter and can be better used to drive the high-speed motor. The loss of switching device in Three-Level circuit is the main factor that affects the performance of inverter, however, the loss calculation method of IGBT is not applicable to SiC MOSFET, and physical model method requires a large amount of calculation. Therefore, in order to accurately and easily calculate the loss of the switching device, the loss relationship between the three-level I-type and T-type topologies is studied by the average method and the loss parameters of the SiC device. Finally, the simulation experiment is carried out by using PLECS software. The experimental results are consistent with the above calculation results, and the feasibility of the loss calculation method is verified.