Focal spot shape and size, beam power intensity distribution, convergence angle and depth of focus which are determined by beam quality and focus system describe the characteristics of focal beam. The influences of these parameters on depth and width of weld seam are studied in theory and experiment. Results shown that enough power intensity is the precondition of realizing laser penetration welding. Depth and width of weld seam rely on energy penetrating into materials. With the same processing parameters, focal spot size decides the acquired power intensity, depth of focus decides the algnment distance and convergence angle has effect on Fresnel absorption in keyhol wall. All in all, beam-focusing characteristics have effect on results of laser welding with different energy acquired at different focus beam cross section. In reality application, laser system, focus system and focal spot position should be selected according to the practical requirements in order to acquire ideal quality of processing.