We evaluate the shock wave profile of beta-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (beta-HMX) by steady flow theory and calculate the force constants by density functional theory. Based on these results, a method to calculate the local phonon states at shock front is developed. We find that the phonon density of states (PDOS) of shock front is non-intersect with the PDOS of crystal, and obtain four important vibrational modes for shock wave: N-NO(2)stretch, NO wagging, CH(2)wagging, and N-NC(2)stretch. According to the vibrational modes, the initial decomposition reactions of beta-HMX at shocked state are denitrification, hydrogen transfer, and ring-opening reactions.