Abstract: It has been difficult for traditional detailed modeling modular multilevel converter (MMC) methods to adapt to the computational efficiency challenges brought by the increasing number of MMC submodules. In this paper, a new harmonic space-state (HSS) based analytical method for MMC simulation is proposed, which can provide transient waveform of each harmonic and achieve large time-steps simulation. First, the state-space equations of MMC considering AC and DC-side coupling effect in time-domain is built, then converted to harmonic-domain by HSS method, and its analytical expression of step response under the open-loop control is derived. Based on this analytical expression, the iteration expression of whole simulation process and its numerical algorithm is proposed. Finally, the simulation results validate the effectiveness of the proposed method.