The influence of three-phase voltage unbalance should be taken into account in the design of voltage sourced converter（VSC）， which may be seriously affected in terms of the operation stability. According to the literature available， the parameters of VSC should be chosen on the basis of the grid voltage magnitude. the relationships between the unbalance factor and the dip depth of grid phase voltage under different unbalance conditions are analyzed. mathematical expressions of the system parameters are deduced from the perspective of the unbalance factor. The rationality of the optimization method is verified by simulation results.