To investigate the effect of micro-defects on the shock response， chemical decomposition， and damage evolution of pentazolate salts， ab initio molecular dynamics method is employed to simulate the dynamics evolution and initial chemical reaction mechanisms for perfect Mn（N₅）₂ crystal and the crystal with 3% vacancy defects under different shock velocities （8， 9， 10， 11 km·s^-1 and 12 km·s^-1）. The calculated Hugoniot curves indicate that the vacancy-containing system exhibits a slightly higher compression ratio under high-pressure conditions than the perfect system. The molecular dynamics results indicate that when shock velocity v_shock<10 km·s^-1， perfect and vacancy-containing system only show a slight （<10%） volume compression and neither of them exhibit chemical reactions within 5000 fs. When v_shock=10 km·s^-1， N─N starts to uniformly rupture within the space of perfect crystal at 512.8 fs， whereas the reaction of vacancy-containing system is advanced to 281.6 fs and the N─N is ruptured near the vacancy. When v_shock continually increases to 11 and 12 km·s^-1， the starting time of reaction for two systems is further advanced and the reaction process is further speeded up. The positive effects of the vacancy on shock sensitivity and chemical reaction process are weakened with the increase of v_shock. The simulated results at the atomistic scale reveal that vacancy defect is one of the early nucleation structures of hot spots. The stress near the vacancy promotes the cascade decomposition of the surrounding pentazolate anion， thereby causing the growth and propagation of damage and ignition of energetic materials.