Abstract: Based on the Fokker_Planck equation,we analytically studied the mechanism of ultrashort pulse laser ablating fused silica,and establish the mathematical model of the fused silica ablation mechanism resulting from the avalanche ionization and multiphoton ionization.The computational result about the critical laser energy density and ablation threshold is in agreement with the experimental result that quantitatively interprets the influence about ultrashort pulse laser on the microscopic processing of fused silica.