Abstract: It is the foundation for engineering, such as laser micromachining etc., of understanding ultrafast laser interaction with materials. Firstly, it was introduced with band electron excitation, Auger recombination effect and two photon excitation etc. to modify present two-temperature model to adapt for the femto-second laser interaction with silicon. Then, the damage threshold was calculated. Thermal and non -thermal damage contributions were analyzed. Finally, two -pulse laser interaction with silicon was discussed. Also, Certain important parameters such as electron density and lattice temperature influence for heat accumulation effect were compared. The laser damage fluence is about 0.25 J/cm2. At this fluence level, thermal contribution dominates damage process, which is identical to experimental findings. With a fluence more than 0.53 J/cm2 non thermal damage mechanism prevails. With two pulses shooting, the heat accumulation effect is predicted, which shows that a time span between two pulses smaller than 100 ns (corresponding to 10 MHz repetition rate) cannot be ignored and it can reduce the damage threshold significantly. It is identified an electron density after the first pulse (lower than 1026/m3) does not significantly change the heating process. However, the lattice temperature after the first pulse (higher than 800 K) can cause serious electron excitation. This research is probably valuable in laser micromachining and laser protection applications.