The invention provides a ball roundness tester and a test method thereof, and relates to a ball testing device and a ball test method. The ball roundness tester comprises a bedplate, wherein a rotary device is arranged below the middle part of the front side of the bedplate; one end of a rotating shaft is connected to the bottom of a tray, and the other end of the rotating shaft passes through the bedplate and is connected to a motor A in the rotary device; two sides of the tray on the bedplate are provided with a left gripper and a right gripper which can clamp and rotate a ball to be tested; the rear part of the tray on the bedplate is provided with a sensing device; one end of the sensing device, far away from the tray, is provided with an adjusting lever; one end of an arm lever is connected to the adjusting lever, and the other end of the arm lever stretches towards the upper part of the tray and is provided with a saddle; and weights B are arranged in the saddle. The tester can test the roundness of every point and plane of the ball and accurately acquire the data value on the surface of the ball by using the sensing device so as to reflect the surface roundness of the ball; and the tester and the test method are scientific, accurate and reasonable.