Scientific experiments show that damage to optical materials is related with factors like laser wavelength, material properties, laser pulse width, etc. The final stage of damage is various, which can be melting, crack and ablation. In an effort to explain these phenomena, we run a semi-quantitative discussion on the optical material damage mechanism regarding the thermal conduction equation, presenting a key parameter that determines the relationship between laser induced damage and laser pulse width τp—the time τ it takes for the inclusion within the optical material to transfer its heat to the body. When τpτ, the damage threshold is determined by laser radiation intensity; when τpτ, the damage threshold is determined by laser radiation energy density.