The modular multilevel converters(MMCs) are attractive topologies for voltage sourced converter based high voltage direct current(VSC-HVDC) transmission system. Asymmetric grid conditions are common encountered in VSC-HVDC system. The resulting voltage sag and power fluctuation from such conditions amplify current stresses on devices, posing a threat to the safe and reliable operation of MMC. This article establishes an equivalent model for MMC under asymmetric conditions, based on which, a decoupling control method for the MMC under asymmetrical operating conditions is proposed.