Abstract: Resistance spot welding is widely used in automobile and motor vehicle body manufacture because of the automation. The destructive experiment is inefficient and can only be controlled by sampling. And destructive experiment efficiency is low, can only through sampling process control. To solve these problems, a water-cooled electrode arm with ultrasonic sensors is designed. In situ monitoring of resistance spot welding quality was implemented and real-time acquisition of ultrasonic echo data of nugget melting and solidification was collected. The correlation between time of flight and nugget diameter was analyzed. Furthermore, the validity of welding parameters is evaluated according to the time of flight.The test results show that when the size of the nugget is larger than the effective size, there is a good linear relationship between the ultrasonic propagation time and the nugget diameter. According to the ultrasonic propagation time, the influence of welding parameters such as welding current and energizing time on the size of the nugget can be determined. This method can effectively monitor the quality of resistance spot welding and provides a new means for welding process control.