Abstract: The welded joints of the rotary tiller frame are subjected to multi axis impact random vibration composite loads induced by non-uniform media during field operations. The local stress field in the weld area exhibits highly nonlinear, non proportional, and non steady state evolution characteristics, which leads to the inability of traditional fatigue analysis methods based on linear static assumptions such as nominal stress method or hot spot stress method to reflect the transient stress concentration effect formed by the propagation and reflection superposition of stress waves under impact loads; A fatigue performance analysis method incorporating the virtual notch radius method is proposed to address the systematic deviation in fatigue damage localization and life assessment caused by the suppression of mesh singularity in the weld toe singular field region of the finite element method. This method first constructs a finite element model of the rotary tiller frame based on Ansys Workbench, and obtains the dynamic stress response of the welded joint under impact load through modal analysis and harmonic response calculation; Furthermore, the virtual notch radius method is introduced to redistribute stress in the weld toe area, and a fatigue life prediction model is constructed by combining the stress concentration factor and equivalent stress amplitude, achieving accurate evaluation of the fatigue performance of welded joints. The experimental results show that under different weld distance conditions, the relative error between the predicted life and the measured value of this method remains between 0.0371 and 0.0213. The prediction accuracy of crack propagation rate is high, and the normalized damage ratio fluctuates less than 0.1 with the change of grid size. It has good computational robustness and engineering adaptability. This method effectively solves the problems of insufficient accuracy and strong grid sensitivity of traditional methods under non proportional multi axis loading, providing theoretical support and technical path for the anti fatigue design and reliability evaluation of agricultural machinery structures.