The invention relates to an ultrasonic nondestructive testing device. The ultrasonic nondestructive testing device comprises a probe, a signal transmitting module, a signal receiving module, a processing module, a storage module and a control module, wherein the signal transmitting module is used for exciting the probe to generate ultrasonic waves; the probe is used for transmitting and receivingultrasonic signals; the signal receiving module is used for receiving the reflected ultrasonic signals from the probe; the processing module is provided with an amplifying circuit, a filter circuit, an analog/digital (A/D) conversion circuit, an envelope signal conversion unit, a scanning unit and a data fusion processing unit; the storage module is used for storing signals processed by the processing module; the scanning unit is provided with a software dynamic gate with continuously set time slots; and the software dynamic gate is used for monitoring ultrasonic signals of a sample to be tested at positions with different depth. The ultrasonic nondestructive testing device overcomes the contradiction between thickness direction subdivision and plate thickness direction coverage rate which cannot be overcome by other detection methods. For a nonuniform-thickness laminated board particularly, the ultrasonic nondestructive testing device guarantees detection rate and guarantees subdivision of the thickness direction.