A nonlinear ultrasonic testing method based on the finite amplitude method is proposed for testing micro fatigue damage, which is hard to be distinguished by the traditional ultrasonic testing. First, the Detection platform which can be used to adjust probe’s acoustic coupling performance is designed to keep the stability of the nonlinear ultrasonic detection signal. Second, the relative nonlinear coefficient of the microdamage zone is extracted under various excitation voltages to investigate the influences of the excitation voltage on the testing effect. Finally, the testing abilities of the nonlinear ultrasonic testing on the microdamage are discussed combining with the metallographic measurement. The results indicate that the relative nonlinear coefficient can effectively characterize the micro fatigue damage, and the excitation voltage is the key detection parameter of the nonlinear ultrasonic finite amplitude method, which decides the detection efficiency and detection resolution.