Laser beam has many advantages, such as good monochromaticity, high brightness, good directivity, etc. Laser technology in laser deep penetration welding process includes a series of complex physical processes, such as the absorption of laser energy by the base metal, the transmission of laser energy through the plasma in the keyhole wall, the change of thermophysical parameters of the base material, the flow behavior of liquid metal in the molten pool, the dynamic fluctuation of keyhole, and phase transformation (melting, solidification of molten pool). In the process of laser welding, when the temperature of the base material in the laser heat source area reaches the critical point of melting and vaporization, the vaporization of the metal produces very high steam pressure, which blows the liquid metal around, thus forming the keyhole. Under the action of recoil pressure, surface tension and other forces, the formed keyhole is always in the process of dynamic fluctuation, that is, the transient change of keyhole. During laser welding, keyhole fluctuates in real time. When the keyhole is closed, welding bubbles will be generated, and welding spatter will be generated on the surface of the molten pool during welding, which will have a great impact on the welding quality. In laser welding, the size of laser spot radius has a direct impact on laser power density, and different laser power density has an impact on the molten pool flow field and the three-dimensional shape of keyhole. However, there are few studies on the influence of laser spot size on the behavior of laser welding pool and keyhole.Based on the above analysis, the heat flow coupling model of laser welding was established based on Fluent software, the effects of laser spot size on the three-dimensional transient behavior of keyhole and the flow field of molten pool during laser welding were studied. The control equation adopted continuity control equation, momentum conservation equation and energy conservation equation. The source term was introduced into the momentum conservation equation and energy conservation equation. Material thermophysical parameters were introduced into the software, including solid density, liquid density, solidus temperature, liquidus line temperature, gas line temperature, latent heat of fusion, latent heat of vaporization, thermal expansion coefficient, heat transfer coefficient, surface tension gradient coefficient and other parameters.When the light spot radius was 0.1 mm (Fig.2), welding spatter appeared in front of the keyhole wall, the bottom of the keyhole was closed, and the outer wall of the keyhole was convex at the middle of the keyhole rear wall and the middle and lower part of the keyhole front wall; When the light spot radius was 0.15 mm (Fig.3), the liquid metal behind the keyhole wall was separated from the liquid column to form welding spatter, and the liquid metal bulge appeared on the surface of the molten pool behind the keyhole wall, and the closure occured at the bottom of the keyhole. When the light spot radius was 0.2 mm (Fig.4), the keyhole was not closed, and the stability of the keyhole was improved.The conclusion of the paper was as follows: (1) With the increase of the spot radius, the depth of the keyhole decreased significantly. When the light spot radius was 0.1 mm and 0.15 mm respectively, the bottom of the keyhole was closed during welding. When the light spot radius was 0.2 mm, the keyhole was closed, and the stability of the keyhole was improved. (2) With the increase of the spot radius, the fluctuation of the maximum flow velocity of the molten pool was relatively small. The average maximum flow velocity of molten pool with spot radius of 0.1 mm, 0.15 mm and 0.2 mm is 10.9 m/s, 10.3 m/s and 7.9 m/s respectively. (3) With the increase of the laser spot radius, the size of molten pool gradually increased, and the average length of molten pool with spot radius of 0.1 mm, 0.15 mm and 0.2 mm was 3.5 mm, 3.8 m and 4.1 mm, respectively.