Based on Kogelnik's coupled-wave theory, diffraction efficiency equation of transmission volume Bragg grating is derived. By numerical simulation, it is analyzed that influence of writing parameters such as refractive index modulation, grating thickness, phase-mismatching parameters, deviation of angles and wavelengths on diffraction efficiency The study is shown that the maximal diffraction efficiency is obtained when phase-mismatching do not occur between the incident waves and the diffraction waves, and diffraction efficiency will be improved at the incidence of Bragg angle when the grating thickness and the refractive index modulation increases. When the incident light met all Bragg conditions,diffraction efficiency of the transmission volume Bragg grating of various thickness and different refractive index modulations is 100%. If it deviates slightly from the Bragg conditions, diffraction efficiency will decline instantly. When the deviation is outside a certain range, diffraction efficiency will drop to zero. Furthermore, we have found that it is not true that the deeper refractive index modulation is, the higher diffraction efficiency.