We set up the three-dimensional transient model of laser deep welding considering the mechanical factors such as evaporative recoil back pressure, the surface tension, the thermal buoyancy and the thermal parameters such as the convection and radiation in and out of pool. In the model, the rotary Gauss body heat source attenuation along depth direction was applied to solving the laser absorption of molten pool, the volume of fluid method was adopted to realize the tracking of gas-liquid interface, and the liquid volume fraction method and enthalpy-porosity technique were used to compute the latent heat of molten metal solidification and the momentum loss in the liquid-solid mush zone. Then the transient temperature and flow field of pool and keyhole were obtained in the laser deep penetration welding of stainless steel with this numerical model. The results show that there are three stages for the maximum temperature in the welding, which are the linear growth, the tending to stable and the small oscillation, and there are two kinds of keyhole orientations, forward and backward, and there is periodic oscillation for keyhole in the welding. The simulation results of final weld formation agree with experimental data of weld cross section well, and the oscillatory behavior of keyhole is also verified by relevant references.