The object of this contribution is to develop the principle of a new type calibration device for water flow rate-the variable water head standard device. Based on the basic principle of fluid mechanics, the authors present the mode of the device, differential equation and the solution under the conditions as follows: the diameter of the pipe and the tank are: d =1.2m, D= 10m, 8m, 5m separately, the height of the tank H_0 = 20m. and there is a magnetic flowmeter in the pipeline. The resuhs of computation demonstrate that in all cases after the valve is opened for a period of time, the change rate of discharge in the pipes is constant. By virtue of it, we might use this device to calibrate the large flowmeters by water.Finally, some other characteristics of the variable water head standard device for large flow rate have been dealt with and theoretically investigated.