Abstract: When the low-voltage distribution network voltage is unbalanced,the generated negative sequence component will cause current unbalance and power oscillation in the solid state transformer（SST） output stage in the traditional virtual synchronous generator（VSG） control. Therefore,this paper proposes a T-type three-level output stage joint control strategy based on VSG to improve output stage power quality. Firstly,the mechanism of VSG current unbalance and power oscillation under unbalanced voltage is analyzed.Secondly,based on VSG control algorithm and instantaneous power theory,a new current reference generator is designed,which realizes cascade control with positive and negative sequence current regulator. This ensures the SST output current balance and constant active/reactive power,and realizes reliable switching of different operating conditions. Finally,the separation method of positive and negative sequence components and the midpoint potential balance algorithm are analyzed and researched,and the control strategy is simulated and verified. The results show that the proposed joint control strategy is effective and feasible,and can guarantee the quality of the grid-connected current and power of the SST output stage under three-phase unbalanced voltage conditions.