In order to solve the uncertainty of the output of distributed power and the effects of load fluctuation, this paper presents a reactive power optimization method based on interval for the grid with distributed power. First, the concepts of interval number and interval arithmetic are introduced and the interval power flow is established. Then, objective function and constraint conditions of interval reactive power optimization model with distributed power are established, and the steps of solving the model with PSO are given. Finally, a modified IEEE 14 node system is simulated through Matlab and a certain power flow for reactive power optimization proves the correctness of the proposed algorithm. Simulation results shows that it can make the grid work in near-optimal state with taking into account the uncertainty of the grid.