Single-shot damage morphology of magnesium fluoride induced by femtosecond laser pulse was studied with scanning electron microscope (SEM). The laser pulses used were 800 nm central wavelength. By the linear relationship between the area of damage spot and the logarithm of the laser energy, we measured the dependence of damage threshold on the pulse duration ranging from 55 femtosecond (fs) to 750 fs. We have calculated the two-photon absorption in the conduction band and improved the multiple-equation model (Phys. Rev. Lett. 92 187401, 2004). The experimental results are well explained by our model.