Based on Kogelnik′s coupled-wave theory and matrix optics, generation of femtosecond double pulses by modulating the thickness of buffer layer of two-layer volume holographic grating is discussed. The diffraction field expressions with a femtosecond pulse reading out the two-layer volume holographic grating are deduced. The relation curve of instantaneous diffraction intensity distribution varying with thickness of buffer layer numerically simulated. Simulation results show that when the buffer layer thickness changes from 4 mm to 11 mm, the diffraction pulse turns into double pulses and the interval between double pulses is linearly proportional to the thickness of the buffer layer. We use the diffraction theory and diffraction field expressions to give an explanation. At last, we discuss time delay of diffraction double pulses with respect to the thickness of the buffer layer by group time delay of periodic media, and it is shown that the slope of the pulse interval with respect to the thickness of the buffer layer is 2 times of that of pulse time delay.