In order tostudy the attenuating character of shock wave in different materials gap, theshock wave attenuation model in gap was established by an explosive similaritylaw dimensionless analysis method. The expression describing shock wave attenuationin gap in PMMA and 45^# steel was obtained. The results show that p_mx/p_m0 is a function of x/dand the p_mx/p_m0 must be equal when x/d is equal matter how x and d change. The expression was tested by experimental results. Theyare in accordance with each other.