With considering the geometry characteristic of laser brazing and analyzing the action features of heat source in laser brazing, an adaptive heat source mode is constructed to simulate the dynamic response of temperature field and flow field of beryllium irradiated by a moving laser. And the shape of fusion zone and thermal cycle curve calculated agree well with the experimental results, it indicate that the proposed model is appropriate and adaptive in simulating the dynamic response of melting pool. Further, influences of latent heat of fusion and butt gap on temperature field and melting pool shape is analyzed by means of the method of numerical Analysis. The results show that the effects of the latent heat are not negligible. In particular, the influence of the latent heat is more violent under moving laser beam action than under stationary laser beam. And the predicted profile of melting pool which taking butt gap into account is more close to that of experimental results.