Reactive power optimization is a nonconvex, nonlinear and mixed integer programming problem, and it is difficult to obtain the optimal solution. In this paper, the problem of reactive power optimization of distribution network is transformed into a convex semi-definite programming( SDP) based on the consideration of three-phase unbalance in distribution network, and the convex problem can obtain the optimal solution. By using the method of sensitivity analysis and the combination of the branch and bound, the mixed programming problem with discrete-continuous variable is soluted. Finally, taking the modified IEEE 33 three-phase power distribution system as an example, analysis results show that the proposed method can accurately solve the nonconvex and nonlinear programming problem.