Site selection of offshore substation plays a vital role for wind farm investment cost. This paper presents a method for optimal substation site selection for offshore wind farms substation. Firstly, the nearest offshore point of an offshore wind farms substations is determined by the shortest distance between the offshore wind farm and the onshore centralized control center. Then according to the position coordinates of each wind turbine, the gravity point of the offshore wind farm is obtained by the gravity method. Subsequently, a straight line is obtained by connecting the nearest offshore point of the offshore substation to the gravity point of the offshore wind farm. Next the substation site is moved along the straight line, and the layout of the current collection system the offshore wind farm is determined by improved Prim algorithm. Then the increased distance of the medium voltage submarine cable and the decreased distance of the high voltage submarine cable are calculated, and the voltage drop of medium voltage submarine cable is checked. Finally, combining the cost of submarine cables and the requirements of construction conditions, the optimal site of the offshore substation is obtained. The results show that the proposed method of this paper is simple, feasible and economically reasonable.