In this paper we report the measurements of the damage threshold and ablation depth for fused silica by laser pulses at the wavelength of 800 nm, with the pulse duration ranging from 45 to 800fs. These results can be well understood by using an improved avalanche model. The model includes the production of conduction band electrons(CBE), laser energy deposition, and CBE diffusion. The evolution of micro-explosion is investigated based on this model.