Abstract: A metal thickness measurement system was presented, in which laser was used for exciting the ultrasounds, while interferometer and piezoelectric transducer(PZT) were adopted for receiving longitudinal acoustic waves. In accordance with mechanism of laser ultrasonic generation, the directivity model for longitudinal acoustic beam and the influencing factors were established and determined respectively, and formulas of two receiving modes were given as well. The effect of defocusing amount on thickness measurement was investigated, discovering phenomena that it would first increase then decrease finally disappear varying with increase of defocusing amount under the same laser pulse energy, consistent with the theoretical results, confirming the effectiveness of the study results.