A ground test method for the Windshield Wiper System is given in this paper. Equivalent Intensity of the Rainfall is raised in this paper, and the intensity of the rainfall that the two front windshields encounter when the aircraft is flying in the rain is transferred to the equivalent intensity of the simulated rainfall in the Ground Test based on the truth that the quantity of the two rainfalls is same. The Ground Test for Windshield Wiper System can be done by calculating the equivalent intensity of the simulated rainfall, the key parameter. The method has been proved to be practical and effective by the engineering practice.