Densification mechanism of metal powder sintered with selective laser was studied. Different parameters were used in the laser sintering of Fe-C mixed powder experiments. The densification mechanism of metal powder was analyzed by observing the microstructure of sintered part. Based on experiment, the relationship between the densification rate and forming parameters was studied and its mathematical model was established. The results show that part of powder melts into liquid phase by low power laser and the densification is caused by powders rearrangement, dissolution and deposition participated in the liquid phase. The affect of laser process parameters on densification rate is decided by the amount of liquid phase produced during laser sintering. With the increase of laser power, the decrease of scanning interval, scanning speed and piece thickness, the densification is improved.