The article describes the relationship of the initial transmission andmax transmission, and the unique choice of output coup ler and initial transmission of Cr4 + ∶YAG crystal. The Nd∶YAG laser is pumped by ring laser diodes. When a pyramid is used as total reflective mirror and Cr4 + ∶YAG used as passively Q switch, the Nd∶YAG laser can output stable laser energy of 200mJ with pulse repeat frequency of 20 Hz and pulse width of 11ns.