In order to improve the quality of laser welding process of 304 stainless steel, a laser welding system with ultrasonic was designed to study the effect of ultrasonic frequency on weld properties. The ultrasonic sound field model was established, and the ultrasonic characteristic frequency and sound pressure distribution were simulated based on COMSOL. Through the real-time monitoring system, the effect of ultrasonic on molten pool area, molten pool flow rate and metal vapor pressure was obtained. Under the condition of different ultrasonic excitation frequencies, the changes of weld microstructure, wear morphology, microhardness and oxidation were studied. The results show that ultrasonic vibration can significantly improve the wear resistance, hardness and corrosion resistance of 304 stainless steel weld. The ultrasonic energy efficiency under the second-order characteristic frequency is higher, the flow rate of molten pool is low and the joint quality is better.