The invention provides a water cooling system of an offshore wind generating unit. The water cooling system of the offshore wind generating unit comprises a water cooling pipeline, a pressure sensor, a water tank and a controller. The pressure sensor is used for detecting the water pressure of cooling liquid in the water cooling pipeline and sending water pressure signals. The water tank is connected with a water replenishing connector of the water cooling pipeline through a water replenishing pipe, the water tank is filled with cooling liquid and a water pump is arranged on the water replenishing pipe. When the water pressure in the water cooling pipeline is lower than a first preset water pressure value, the controller controls the water pump to start. When the water pressure in the water cooling pipeline is higher than a second preset water pressure value, the controller controls the water pump to stop. The controller is connected with the pressure sensor. According to the water cooling system of the offshore wind generating unit, on the condition that the needed water replenishing amount is very small, the controller and the water pump can be used for automatically completing the water replenishing task of the cooling system, water in the water tank can be used for a long time, when the water in the water tank cannot meet the water replenishing requirement, maintaining is conducted once, so that the maintaining frequency of the offshore wind generating unit can be reduced, therefore, the halting frequency of fans is reduced, the maintaining time is shortened, the maintaining cost is lowered, and shut-down power generation losses of the unit are reduced accordingly.