According to the principle of automatic synchronization, the screening machine produces vibrator excitation force into a direction of fixed angle. Two eccentric blocks are of same quality, and turn towards synchronous direction. In each of the centrifugal force generated at the moment of the component in the direction of vibration is always overlaid. The normal components always cancel each other out, thus forming a single excitation force along the direction of vibration, making the screening box do reciprocating linear motion.