Laser beam welding has gained increasing application in the industry, because of its high ratio of penetration deep to weld width, low heat effects and high welding speed. Keyhole effects are the essential characteristic in laser deep penetration welding. A clear stable keyhole was observed with a high-speed camera in laser welding of GG17 glass. The effects of defocus and welding speed on the size and the appearance of the keyhole and the welding pool were experimentally studied. Under the assumption of the keyhole per thin layer being cylindrical, a heat transfer model was proposed to obtain the temperature distribution and the fluid velocity distribution around the keyhole by a finite element method based on the keyhole observed. Experimental study and simulation show that temperature gradient on the front keyhole wall is steeper than that on the rear, the mass flow rate in the welding pool is about ten times the welding speed, and a new technology for quantitative study of keyhole effects in deep penetration laser welding is developed.