A testing device for testing whether planeness of top and bottom surfaces of an object is within an allowable range includes a rack, a motion mechanism, a transmission mechanism, a control mechanism, a testing mechanism, and a display. The motion mechanism is slidably connected to the rack and supporting the object. The transmission mechanism is fastened to the rack and the motion mechanism. The testing mechanism includes two separated rows of sensors. The object extends through the testing mechanism, between the rows of sensors. The sensors above the object measure distances between the sensors and corresponding dots of a top surface of the object. The sensors below the object measure distances between the sensors and corresponding dots of a bottom surface of the object. The sensors send signals about the distances to the control mechanism.