Abstract: In order to improve the operating characteristics of the control system of modular multilevel converter based high voltage direct current transmission（MMC-HVDC）system with an asymmetry fault on the AC side,the negative-sequence current suppression and DC voltage stabilization control strategy under the grid unbalanced fault is studied.For the problem of double-order inner current control system requiring rotational transform voltage and current and coupling between the d axis and the q axis,the second-order complex filter is designed to extract the positive and negative sequence component of the grid voltage.And the idea of voltage compensation principle is used to design inner negative sequence voltage controller which uses the extracted negative sequence component,thus effective improvement has been made and the structure of the control system simplified for the double-order inner current controller.In addition,to achieve the control of asymmetric fault DC voltage stabilization,on the basis of the low-frequency continuous model,the bridge arm power and the sub-module capacitor voltage are deduced with the double frequency component of negative and zero sequence fluctuations.Consequently,a secondary zero sequence compensation controller is designed.The proposed controller is simulated in the environment of PSCAD/EMTDC and simulation results show that the proposed controller can effectively inhibit the negative sequence current and stabilize DC bus voltage.