Abstract: When the power grid is short circuited, the fan needs to provide reactive power to the system to support the voltage recovery of the power grid. If only positive sequence reactive power is injected into the system in case of asymmetric fault of weak current network, the voltage rise of non fault phase will be caused while the fault phase voltage is raised, which will aggravate the unbalance of three-phase voltage at the fan end. Therefore, it is necessary to inject certain negative sequence reactive power into the fan to suppress the voltage rise of non fault phase. In order to further optimize the support strategy of fault phase voltage of permanent magnet direct drive fan under asymmetric fault, we first established the mathematical model of fan supply end system and phase locked loop of positive and negative sequence decoupling control. Then the coupling relationship between the voltage and active and reactive power of fan terminal was deduced. The paper proposed that the maximum fault phase voltage at the end of the machine is the optimization objective, and the constraint condition is that the maximum value of three-phase current does not exceed the current limit of the converter. In this case, the fan can generate reactive power and output a certain active power, which is more conducive to the support of non-fault phase voltage. According to the simulation model of a wind power transmission terminal system in North China in Matlab/Simulink, the feasibility and effectiveness of the control strategy were verified.