Using a specially designed experimental setup, we obtained a clear stable keyhole with a high-speed camera. A heat transfer model with a cylindrical surface heat source has been developed under the assumption of the keyhole per thin layer being cylindrical. The model is numerically solved by finite element method, the temperature field around the keyhole and the heat flux lost to the keyhole wall can be obtained. The effects of such factors as the shape and the size of the keyhole, the welding speed on the shape of the melt pool are studied. By comparing the laser intensity absorbed on the keyhole walls with the heat flux lost there, the mechanism of energy balance on the keyhole walls is investigated.