The invention discloses a water supply device of an after-rolling cooling system, which comprises a water pump and is characterized in that: one path of the water outlet of the water pump is connected with a cooling system through a pipe; the other path of the water outlet is connected with a high water tower through a pipe; and the high water tower is provided with an overflowing opening. The pipe connecting the water outlet of the water pump and the cooling system and the pipe connecting the water outlet of the water pump and the high water tower are both provided with a valve. When the water supply device works normally, both a valve (7) and a valve (8) in the pipes are opened; when the flow of the water supply pump is bigger than the actual water volume for superfast cooling, excessive water flows through the valve (7) to return to the high water tower and overflows out of the overflowing opening of the high water tower; and when the flow of the water supply pump is smaller than the actual water volume for superfast cooling, the water level in the high water tower falls to ensure the water supply capacity of the water pump is bigger than the maximum water volume for superfast cooling, and thus, the pressure of the whole water supply system can remain constant all the time.