The 1064 nm high reflectors are prepared by e-beam deposition, and via laser conditioning with raster scanning on certain areas, plasma scalds are formed on the surface. The beam quality testing system is built. The spatial intensity distribution of laser beam reflected by the samples with or without plasma scalds is recorded. The power spectral density of laser beam is calculated with periodogram method. The results indicate that the peak of the power spectral density corresponds to the peak intensity of the transmitted beam which is modulated by plasma scalds. Therefore the modulation of transmitted laser beam can be analyzed and characterized using the power spectral density calculated from the distribution of the laser beam.