An experimental system to observe the inner phenomenon of keyhole and molten pool through glass plate by high speed camera and spectrometer in high power density laser welding was set up. Two circular flows in the molten pool were observed by high speed camera, which transferred the mass from the front to the rear of the keyhole to keep the mass balance. Temperature distribution in the molten pool was firstly detected by spectrometer, which indicated that the circular flows acted as the cooling system to take heat away from the keyhole. The porosity formation process was also observed and the mechanism was discussed.