Analysis model of the fiber gratings with random spaced index modulations was established. The random spaced index modulated fiber grating can be treated as the random combinations of long period uniform gratings and short period uniform gratings. The reflective and transmissive characteristics of a random fiber grating with the averaged random modulation space 20 μm were calculated with transmission matrix method and beam propagation method. The results confirmed the coexistence of the coupling between the forward and backward propagated fundamental core modes and the coupling between the fundamental core modes and cladding mode. A random fiber grating with 500 modulation points was fabricated by femto-second laser, the spaces between the adjacent random modulation points were about 10~20 μm. The reflective and transmissive spectra of the grating were measured experimentally. The experimental results further validated the numerical analysis.