It is proved that the employment of two-fold or four-fold speed-up deviee is necessary for a shutter Q-switch and there is a slight flicker of the laser beam in the cavity. The mechanism of multi-pul ses generation is explained. The numerical calculation for typical cases is completed with a computer. It is proved that the performance of the shutter Q-switch will be improved by placing- the prism in a closed cell with gas of suitable refractive index.