The influence of laser parameters on laser ultrasonic signals is studied. Firstly, the physical mechanisms of ultrasonic generation by laser pulse and the influence of laser parameters on the ultrasonic signal are analyzed theoretically. And an experimental apparatus is designed and built to verify the theoretical analysis results. In the experiment the ultrasonic is generated with pulse laser and optical attenuation unit, and then the detection of laser ultrasonic signals is achieved by a piezoelectric ultrasonic transducer. In the experiment, the influence of the pulse laser energy and laser spot diameter on ultrasonic signal is studied with an aluminum plate. Through theoretical analysis and experimental verification, the laser power density can be affected by the laser spot diameter and laser energy, which will affect the generated ultrasonic amplitude.