Abstract: Compared with XRD, blind-hole method, and magnetic elasticity method, the use of ultrasonic testing has significant advantages in residual stress and online stress detection for in-service pressure vessels and steel structures. However, the existing ultrasonic stress equipment generally has poor stability and portability. This paper introduces a self-developed high-precision portable split-type ultrasonic stress testing equipment based on WiFi communication technology. The hardware includes an ultrasonic acquisition module, ultrasonic probe, and microPC upper computer, while the software includes setting, signal acquisition, signal optimization, stress solution, and calibration modules. It can also be powered by a fast-detachable universal sliding battery pack. At the same time, the residual welding stress in different positions of Q235 welded plates in parallel and perpendicular to the weld direction was studied. The results show that the residual stress in the parallel weld direction remains stable, and the residual stress gradually decreases as the distance from the weld position increases. Preheating before welding can effectively reduce the residual stress. The residual stress of the perpendicular weld is smaller than that of the parallel weld, with its stress value equal to the average value of the residual stress of the parallel weld detected in the same position.