The reactive power optimization in distribution networks with distributed generators is a nonlinear and non-convex problem in essence and one hard to get an optimal solution.In light of the radial operation characteristic of the distribution network，this paper develops a three-phase reactive power optimization model based on branch power flow equations.Then the original reactive power optimization model is converted to a mathematical programming form with a convex feasible zone based on the second-order cone relaxation technology.Further，the model is extended to a mixed integer second-order cone programming model by taking the discrete compensation devices（such as capacitors）into account.The model can easily be solved by existing optimization solvers.Numerical tests on an IEEE33-bus and IEEE123-bus distribution system show that the proposed method is capable of efficient global optimal solution.