The finite difference time domain method (FDTD) was used to research the influence of the micro/nano fiber on the beam quality. The micro/nano fibers were made by using fused-tapered technique. And using CCD, we have measured the beam propagation factors M2 of beams passing through the tapered micro/nano fiber and the single mode fiber, respectively. The experimental results indicated that, compared to the beam propagation factor M2 of beam propagating along the single mode fiber, M2 of beam propagating along the tapered micro/nano fiber is better. The experimental results are consistent with the FDTD calculation results.