Abstract: Based on the reduced growth rate equation of electron density, a theoretical model is established to describe the change of the conduction band electron density of fused silica with the laser pulse duration. Using the concept of the critical electron density, a scope of the laser-induced damage of fused silica is calculated with the pulse width from 150 fs to 10 ps. The analysis shows that the avalanche ionization still takes a leading role from 5 ps to 10 ps. Instead of electrons provided by the fused silica itself, the initial electrons produced by the photoionization make a contribution to the impact ionization. It reaches a balance between avalanche ionization and photoionization when the pulse width shrinks to 4 ps. After that, the photoionization plays a main role in the growth of the conduction band electron density. Through the fitting curve, a new pulse width law is obtained to satisfy the laser-induced damage threshold changing from 150 fs to 10 ps. The results demonstrate that the threshold is proportional to tp0.38(tp is the pulse width), and changes to tp0.34 when the temperature dependence is concerned.