A three-dimensional computational fluid dynamics model is developed in this paper to analyze the symmetry of weld geometry, temperature field distribution and flow field distribution and the homogeneity of alloy distribution during the laser welding of dissimilar materials of low carbon steel and stainless steel under the different welding conditions. The characteristics of weld morphology, temperature field, flow field and mass transfer are calculated. The simulated result is compared with the experimental result and the simulation result shows good agreement with the experimental result. It is indicated that with the increase in laser power, the depth and width of weld are gradually increasing and the geometry of weld tends to be more symmetrical. The fluid convection is enhanced under the higher power welding condition, which results that the alloy distribution in the molten pool is more homogeneous and the upper and lower width difference of the weld is decreasing. The proposed numerical simulation model can accurately predict the weld morphology and alloy composition of fiber laser welding of dissimilar materials, and provide the effective method for selecting optimal process parameters to obtain the desired weld bead with excellent performance, which has practical guiding significance for improving the weld quality of laser welding of dissimilar materials.