The invention provides an installing method of tensioning hydraulic cylinders of a water insulating casing pipe of a drilling platform. The installing method is used for installing the tensioning hydraulic cylinders on tackles; and a platform body of the drilling platform is provided with two parallel tackle bases and the two tackles installed on the tackle bases in a sliding manner. The installing method of the tensioning hydraulic cylinders includes the steps that the tensioning hydraulic cylinders are placed on a ship and sail to the positions below the tackle bases; and the plurality of tensioning hydraulic cylinders are sequentially hoisted upwards through a wire rope sling and are vertically installed at the lower ends of the two tackles. According to the installing method of the tensioning hydraulic cylinders, hoisting and installing are performed from the position below the platform body, limitation by the testing sequence of various devices on the platform is avoided, a special installing device is omitted, installing operation can be completed just by arranging the ship and the wire rope sling, and the installing cost is greatly lowered.