In the laser ultrasonic defect testing experiments, After data acquisition system collected ultrasonic signal amplitude are generally small. In order to increase the amplitude of the ultrasonic signal, we generally have collected signal denoising or change the pulse laser related factors to improve the detection accuracy of the experiment. In this paper,through the tests based on the laboratory’s excellent laser ultrasonic testing platform, we present that changing the laser spot size with the density of the laser power altering can have impact on the laser ultrasonic signal characteristics.Analysis of the spectral characteristics and spectral components of the ultrasonic signal, we show that the density of the laser power can not only affect the laser excitation mechanism, but can also affect the laser ultrasonic signal amplitude and the spectral components of the laser ultrasonic signals.