Laws for temperature fields and cooling rate during laser welding have been studied by numerical simulations, in order to explore the mechanism of successful laser welding of the Ti40Zr25Ni3Cu12Be20 bulk metallic glass (BMG). Taking the effect of convection and radiation on heat transfer into account, the thermophysical properties were detected as a function of temperature. Heat source model with the cylinder and Gaussian was selected. Based on the model, the temperature fields of the Ti-based BMG at different welding parameters were calculated. Combined with the results of experiment and simulation, the microstructures from welded zone and heat affected zone have been analyzed. The mechanism of successful laser welding of Ti40Zr25Ni3Cu12Be20 BMG has been discussed from heating process and cooling process. The results obtained from the microhardness tests of completely amorphous joint indicate that the heat affected zone (HAZ) has higher hardness than those of the parent metal and weld zone (WZ). The phenomenon was studied theoretically from the relationship of glass transition temperature, cooling rate and yield strength. The experiment was also done to validate the accuracy of the analysis