Abstract: Non-destructive evaluation of fatigue damage of train spring was carried out by using nonlinear ultrasonic method.Using the nonlinear ultrasonic measurement system,longitudinal ultrasonic wave was excited and acquired after propagating through the train spring under fatigue load.The relationship between the nonlinearity parameter and fatigue cycles was established after signal process for the received ultrasonic waves.Experimental result shows that increase of the nonlinear parameter was relative slow when the fatigue cycle was less than 150 million times because the micro-structure is not sufficient enough to have obvious nonlinear accumulation on ultrasonic waves.When the fatigue cycle was between 150 and 300 million times,the nonlinear parameter increased obviously with the fatigue cycle,which showed that fatigue damage aggravates during this period.Increase of the nonlinear parameter slowed down when the fatigue cycle was more than 300 million times.This research demonstrates that fatigue damage of train spring is closely related to the nonlinear parameter,and it is expected to provide a promising way for the characterization of material fatigue damage and degradation effectively in a nondestructive way.