In this paper, the historical developments of laser beam welding (LBW) are briefly reviewed. After extensive analysis, it is shown that LBW is very suitable for vacuum technology. A model of heat conduction is suggested. In this model, the surface reflectivity is assumed to vary continuously during the melting process. A fictitious temperature method is developed. This method has the merits of clear physical conception, high precision, simplicity and effectiveness. It is especially suitable for computing the unsteady temperature field with two-point forward difference scheme. A three-dimension finite element program is designed for the latent heat processing. The program is quite versatile, applicable for different laser energies, pulse width, material and thickness. The computation results show good agreement with the analytical results appeared in the literatures. The results also show that the temperature field of material surface is approximately of Gaussian distribution, the depth of penetration and the radius of fusion decrease almost linearly with the increase of pulse width, or with the decrease of pulse energy. The temperature in the centre of material surface is directly proportional to the pulse energy and inversely proportional to the pulse width. The variation of material thickness has great influence on the depth of penetration, but has less influence on the radius of fusion. The parameters of LBW are closely related with the thermal characteristics of material. Experiment results obtained with the laser welder type DW-1 is consistent with the computation results.