Abstract: The physical mechnism and characteristics of the blow off from a LY-12 aluminum target irradiated by a pulsed Nd: glass laser beam are investigated in this paper. The laser beam consists of a series of pulses with a duration of about 2μs for each and a time interval of about 5-10 μs between two adjacent ones. The blow off process shows a time-dependent behavior similar to that of the laser beam irradiating the target. For the beam of mean power-density of 10⁷W/cm², some spikes are higher than 10⁸W/cm². The blow off is measured by framing and streak photography. Besides the vaporization and slower blow off, it is found that there exists a vapor and plasma blow off process directed upstream to the laser beam with a speed of 7.7mm/μs, which should be the laser absorption wave phenomenon. In addition plasma shielding effects seem to be observed.