The design of the cooling water system on wind turbine service unit with DP-1 and DP-2 dynamic positioning grades is quite different. This paper introduces the cooling water system on a 1 200 t self-elevating and self-propelled wind turbine service unit, and analyzes the running situations of the cooling water system under the different operation conditions. In order to meet the requirements of the cooling water under the different operation conditions, the cooling water systems of the ship with DP-1 and DP-2 dynamic positioning grades are designed and calculated respectively.