Modular multilevel converter (MMC) has the characteristics of multi-frequency coupling, and there is a risk of multi-frequency instability in the power system. Different from the existing modeling method, linear time-periodic (LTP) time-domain analysis method does not ignore the multi-frequency components in the modeling process, nor does it make complex multi-frequency components expansion, which is suitable for stability analysis of MMC. In this paper, the fundamental frequency current controller and time delay of controller are considered, and LTP time domain model of the MMC grid-connected system is established to study the influence of the controller parameters on the system stability when the MMC is connected to the strong and weak grids. The simulation model is built by MATLAB/Simulink to verify the correctness of the analysis results.