Abstract: This paper aims to construct a prediction model of ultrahigh cycle fatigue strength of TC4 welded joints. The ultra-high cycle fatigue test of TC4 welded joint is carried out under the stress ratio of −1, and the results show that the S-N characteristics of TC4 welded joint show a continuous decreasing trend, and the fitting effect of P-S-N curve is good. According to the observation of the crack morphology on the surface of the fracture by scanning electron microscope, when the fatigue life is less than 1×10⁶ cycles, the fatigue failure forms are both surface failure and internal failure. When the fatigue life is higher than 1×10⁶ cycles, the fatigue failure mode is mainly internal failure. Finally, based on the failure mechanism, the fatigue strength prediction model of TC4 welded joint was established, taking into account the size of internal defects and fine grain region. Compared with the fatigue strength predicted by other models, the prediction error of the new model is the least, and the prediction result is more accurate.