This paper presents a three-phase state estimator based on linear least square method for the distribution network to solve the problem which is brought by the distributed renewable energy with random output power accessing to distribution network and the limitation in the number of measuring devices. Firstly, the voltage estimation model of distribution network is established based on the three-phase voltage drop calculation, then the expression of the estimator is obtained according to the linear least squares method. The influence of the wind power accessing on the performance of estimator is analyzed. The optimal placement of a certain number of measuring device is established with the decision variables, objective function and constraints. The validity and effectiveness of the designed estimator and the optimization method are verified by the simulation carried out in a IEEE 33 nodes system. This paper provides a theoretical and practical method for distribution network state estimation with distributed energy accessing.