A large number of PVs can produce security and economic influence for distribution system, for its probabilistic and intermittent output. The solar radiation which PV component receives is easily influenced by clouds. The clearness index can reflect this. So a PV output stochastic model is built based on clearness index. In this paper the probabilistic output of PVs and loads are both considered and expressed by stochastic variables. A random expectation model has been built considering annual saved environment cost. The objective function is the minimum expected value of annual investment and operation cost. The operation constraints are to ensure the reliability and economy of power system. The solution algorithm is PSO combining with probabilistic power flow. Qualification rates of bus voltage and line power flow are examined after planning. At last the calculation example manifests this planning model can get good result under the condition of reflexing the probabilistic output of PVs and randomness of power loads.