Abstract: Based on the fact the optical field distribution of the transmitted beams of a multi-mode fiber can be treated as Gaussian distribution,the optical fields of a fiber bundle made up of multi-mode fibers are incoherent.The intensity distribution of a fiber bundle coupling LD output beams transmitting space coupling system is numerically simulated and experimentally studied.The simulation results are in good agreement with the experimental ones.Both of them show that for a thin convex lens,the maximum intensity value and the minimum beam size are obtained in the focal plane,and that with the increase of the focal length,the beam size increase linearly and center intensity decrease dramatically in the focal plane.