The ablation mechanism and ultra-fast dynamics property of LiF crystal are studied. By means of SEM and AFM the damage features induced by femtosecond laser are observed. Based on the logarithmic dependence of the ablated area on laser pulse energy, the ablation threshold fluence of LiF crystal is calculated. Meanwhile, stretching the pulse duration up to 800 fs with dispersive material, the threshold fluence of LiF is got as a function of pulse duration. By use of pump-probe technique, the dependence of reflectivity of LiF crystal on delay time is got while irradiated by 800 nm femtosecond laser with 70 fs and 500 fs pulse duration respectively. Then based on the avalanche mode and the relationship between the reflectivity and the dielectric constant, a calculation in theory is made. The theoretical calculation results agree well with the experimental results. Finally the variety of the density of conduction band electrons in the ablation process is discussed and the experimental result with avalanche mode is interpreted.