Abstract: As an important functional part of ultrasonic assisted welding trolley, magnet mounted frame plays a key role in the stability, safety and reliability of ultrasonic assisted cavitation system of welding trolley. Based on the variable density method and the optimization criterion iteration method, the topology optimization analysis of the welding trolley frame is carried out. With the minimum flexibility (minimum external force work) as the objective function and the volume fraction ≤ 54% as the constraint condition, a new scheme of the frame structure is obtained. Combined with the fatigue failure mechanism of frame material, the fatigue strength prediction model of frame material is established. The results show that the weight of the optimized frame model is reduced by 37.5%, and the optimization target is achieved well; Compared with the original test data, this predicted fatigue strength error is small and reliable. Therefore, this method provides a good idea and reference for the structural design and fatigue strength prediction of ultrasonic welding trolley.