Abstract: The analysis and estimation for fatigue life of welds have always been difficult in the design and manufacture of rail transit vehicles. The existing estimation methods are generally based on finite element simulation, whose results are often not validated effectively. An ultrasonic inspection and evaluation method for fatigue life estimation was proposed. Nonlinear ultrasonic inspection was performed on the specimens with welds in different fatigue stages. An equivalent nonlinear coefficient was proposed to evaluate the fatigue damage, and the relationship between this parameter and the fatigue life was utilized to construct a database. The fatigue life of the friction stir welding joints was evaluated based on this database according to the Bayesian Theorem. This method has the advantages of low cost and high reliability, and is expected to help promote the technical innovation of fatigue life evaluation methods.