The invention discloses a pre-wetting device for automatic ultrasonic detection. A collecting tank of the pre-wetting device is arranged on the top face of a base, a bottom plate is located in the collecting tank, and bolts penetrate through the bottom plate and the collecting tank to fix the bottom plate to the top face of the base. A box is arranged on the top face of the bottom plate through bolts, an opening is formed in the bottom face of the box and communicated with the collecting tank, through holes are formed in the top face, the front face and the back face of the box respectively, the through hole in the top face of the box is connected with a stop valve, a fluid control valve and a quick connector in sequence through a pipeline, an annular pressing plate, an annular sealing plate and a scraping plate are arranged in the through holes in the front face and the back face of the box respectively in sequence, an annular channel coaxial with the through holes in the front face and the back face of the box is formed in the box, a plurality of nozzles are arranged on the inner ring of the annular channel at intervals, and the input ends of the nozzles are communicated with the pipeline connected with the through hole in the top face of the box. The pre-wetting device can effectively scrap off impurities on the surface of a workpiece, prevent foreign matter from entering precise detection equipment and prolong the service life of the detection equipment; the pre-wetting device can pre-wet the detected workpiece, guarantee the coupling effect and improve detection precision.