The invention provides a clamp for vibration testing of a square plate simply supported on four sides. The clamp comprises a base, adjustable locking parts and pressing strips. The base comprises a front support steel channel, a rear support steel channel, a left support steel channel and a right support steel channel which are fixedly connected into a square frame, the front support steel channel and the rear support steel channel are arranged oppositely, and the left support steel channel and the right support steel channel are arranged oppositely. A transverse rib is arranged between the front support steel channel and the rear support steel channel, and the distance between the front support steel channel and the transverse rib is equal to that between the rear support steel channel and the transverse rib. Two longitudinal ribs are arranged on the transverse rib in an equally spaced mode. The two sets of adjustable locking strips with the same length are arranged on the upper surface of the front support steel channel, the upper surface of the rear support steel channel, the upper surface of the left support steel channel and the upper surface of the right support steel channel in parallel respectively. Grooves with the same height are formed in the inner sides of the adjustable locking strips. The pressing strips are arranged in the grooves. The side faces of the steel channels of the base are coated with anticorrosive paint after being subjected to rust removal. Sand blasting is conducted on the surfaces of the adjustable locking strips. The clamp is high in rigidity and clamping force. During vibration testing, the movement of a vibration table top is transferred to the square plate in a distortionless mode.