Abstract: In order to study the influence of defocusing amount on laser ultrasonic signal amplitude, thickness measurement experiment was carried out. Aluminum test blocks in different thickness were irradiated by the same pulse laser while using plano-convex spherical lens whose focal length was 100mm. The lens was fixed at a 5-D optical stage and the horizontal axis knob was adjusted to change the defocusing amount. The ultrasound signals were received by piezoelectric probe with 5MHz center frequency and the ultrasonic signal data of all test blocks under each defocusing amount were recorded. The thickness of each aluminum test block was obtained according to the time interval between the signals. The results indicate that relative errors between the measurement value and the actual value are in the range of 3% and the maximum signal amplitude can be obtained when the defocusing amount is -10mm under the same laser irradiation.