The high-speed photography has been employed to realize in situ observation on the formation and evolution of the molten pool in the process of laser rapid forming. The results show that the substrate begins to melt and produce a liquid molten pool with the laser beam scanning on it. After a rather short time (about 1.0 s) the molten pool depth increases to a certain value while the molten pool length fluctuates surrounding an invariableness figure. The depth and width of the molten pool decrease gradually with a constant flow rate powder being fed into the molten pool. And the lifespan of the molten pool turns to be short. Simultaneously the tail of the molten pool runs up that means the thickness of the cladding layer increases continuously with the powder ejecting into the molten pool. The angle between the normal of the free surface of the molten pool and the laser beam axis increases from a few degrees to 20～30 degree. There is an “island heave” emerging and vanishing seasonally at the tail of the molten pool free surface by in situ observation. This is the result of forced convection on the fused liquid of the molten pool induced by surface tension gradient, which has been proved by numerical simulation.