Based on the theory of quasi-three-level rate equation, the theoretical analysis and simulation of output power and threshold power of Yb∶YAG laser have been obtained. The results of the relative curves on the output power and threshold powerwith the spatial characteristic parameters, crystal lengths, transparency rate, and loss of round-trip of laser-diode-pumped all-solid-state Yb∶YAG laserwere presented. The high power output and low threshold powerwere calculated when a 1. 5 rate of resonant beam to pump beam facula, 0. 2 - 0. 4cm of crystal lengths, and 1. 5 rate of transparency in the paper.