Using Kogelnik's equation of diffraction efficiency of monochrome light from thick hologram gratings,an expression for calculating the diffraction intensity spectrum of a photorefractive volume hologram under ultrashort-pulsed light illumination was derived. Then total diffraction intensity and diffraction efficiency were deduced. Study showsd that the diffraction intensity spectrum,total diffraction intensity and diffraction efficiency were in close connection with grating spacing Λ,grating thickness d,and readout pulse width Δτ. Through modulating these parameters,different extent of filtering for the incident ultrashort pulse can be realized. The results can be utilized to study the shaping of ultrashort pulse.