Based on timing characteristics of distributed generations(DG) output and loads, a planning model for optimal site selection and capacity determination of DG in distribution network, in which environmental cost is taken into consideration, is built. In the planning model, the minimization of annual cost of distribution network, including the cost of distribution network construction, distribution network losses, power purchase cost and the running cost of distributed generations, is taken as objective function, and the forward and backward sweep algorithm including PV node is adopted to judge whether the built planning model can meet power flow constraint or not and to calculate the distribution network loss. Meanwhile，to remedy the premature convergence of PSO, the improved particle swarm algorithm based on classification and correlate is proposed to solve the built model. Finally, the reasonableness of the built model is verified by planning results of a 13-bus distribution system.