The utility model relates to a steel plate wiping and automatic blowing device of an online ultrasonic defect detection system, which belongs to the technical field of the online ultrasonic detection of the steel rolling production line. The device comprises detecting gratings, a water wiping device, a water removing and blowing device, a magnetic valve and a steel plate conveying system, wherein the water wiping device and the blowing device are arranged on the gap between two roller-ways of the steel plate conveying system, and two detecting gratings are respectively arranged on the right sides of the water wiping device and the water removing and blowing device. The detecting gratings are connected with a coil of the magnetic valve through a signal wire, the magnetic valve is respectively connected with a cylinder of the water wiping device and a blowpipe of the water removing and blowing device through a plastic windpipe, the lifting of the water wiping device is controlled by the cylinder, which emits signals and transmits the signals to the magnetic valve by the detecting gratings the supplying of compression air is controlled through the closing and the opening of the magnetic valve, and then the lifting action of the cylinder is realized, and the work of the water removing and blowing device is also realized through the control of the magnetic valve. The utility model has the advantages that the conveying of the roller-way and the normal cutting of double side shears and dividing shears are guaranteed, and the smooth production in winder is guaranteed.