Laser is a usual light source in many measurement applications, because it has good coherence. To measure the light transmission of a large-core fiber, we designed and setup a testing system using a He-Ne laser. The light from the laser could be focused to an ideal point to incident the fiber. On the other hand, the output spot from the fiber suffers serious coherent noise, which is called as speckle. Even using multi-image average or removing mask filter, we can’t reduce the influence of the speckle remarkably. So we made a mode-disturbing device to vibrate the fiber at a certain frequency and amplitude. To check the effectiveness of the fiber vibrating method and to find the best parameters for our fiber mode-disturbing device, we set different working frequencies from 0 to 80 Hz. And we set the CCD exposure time at 100ms, 200ms, 500ms and 800ms. According to the experimental results, the speckle is much more sensitive to the working frequency of vibrating the fiber than the exposure time. After comparing different frequencies, 70Hz is chosen as the optimized frequency to effectively suppress the speckle, decrease the speckle contrast. The output of the large-core fiber could fit to a 2-D Gaussian function. So we can measure the diameter of the spot based on the fitting result. Using this mode-disturbing system, we measured the focal ratio degradation of a large-core fiber and studied on the bend effect of the fiber.