Abstract: Power loss and phase unbalance of three-phase voltage in active distribution networks（ADNs）become more and more serious due to the increasing unbalance of three-phase loads. As a kind of power electronic equipment which can replace the traditional tie switch, soft open point（SOP）provides a potential solution to reduce power loss and alleviate the unbalance problem.Therefore, a three-phase four-wire back-to-back voltage source converter is introduced as the topology of SOP, and the corresponding phase-switching control strategy is designed for the topology. On this basis, a multi-objective operation optimization model of ADNs based on three-phase four-wire phase-switching SOP is proposed to reduce the power loss and alleviate the voltage unbalance. The symmetric semi-definite programming algorithm is used to transform the original non-convex nonlinear model into a semi-definite programming model which is easy to solve by convex relaxation. This model can be used for three-phase decoupling and analysis of three-phase power grid. Finally, a case study is carried out in an improved IEEE 123-bus system to verify the feasibility and effectiveness of the proposed model and its solving algorithm.