Based on the volume grating coupled wave theory,temporal and spectrum expressions of a non-absorption transmission volume grating read out by a femtosecond pulsed laser beam are deduced. Then their changes with the refractive index modulation are discussed. It is showed by numerical simulation that,when the refractive index is increasing in a certain range,at first the temporal diffracted pulse will be one,then the center of the pulse will decrease and split into two pulses,then into three pulses. If increasing the refractive index modulation further more,the number of pulses will return to two,then to one. The above processes will be repeated if the refractive index modulation increased to another certain level. Moreover,if the refractive index modulation changes from 1.0×10-3 to 1.0×10-2,the interval between two diffraction pulses will increase with the increasing of the index. And the simulation of the changes of diffraction intensity spectrum with the refractive index are also mode.