The present disclosure discloses a power conversion system and a communication method for transmitting common mode information in the power conversion system. The communication method comprises the following steps: (a) providing at least two power conversion cells; (b) generating, by each of the power conversion cells, an AC harmonic according to a first electrical signal at the first terminal of the power conversion cell, wherein an amplitude of each AC harmonic represents first information of the power conversion cell, and all the AC harmonics are at the same frequency; and (c) injecting the AC harmonic generated by the corresponding power conversion cells into the first terminal of the corresponding power conversion cell and applying a closed-loop suppression to the AC harmonic generated by the corresponding power conversion cell, and controlling the resonance control unit to output a second electrical signal related to the first information.