Abstract The initial velocity of the secondary fragments of reinforced concrete slabs under blast loading is the basic data for the analysis of damage effect and it can also serve as important basis for the damage evaluation. In this study, three different types of reinforced concrete target slabs with different thicknesses were designed and prepared. These target slabs were investigated by local damage effect tests, the flying processes of the produced secondary fragments were recorded, and the velocities of the secondary fragments corresponding to different collapse coefficients were calculated by fragment back tracking. The results demonstrate that the initial velocity of the fragments was lower at a larger collapse coefficient. Based on the dimensional analysis results, the empirical formula of the initial velocity of the secondary fragments was derived. The errors between the calculated values according to the empirical formula and the experimental data were compared and found to be within 16% at a collapse coefficient in the range1.20–3.20m/kg1/3. Thus, the derived empirical formula can be used for predicting the initial velocity of the secondary fragments produced during the collapse of reinforced concrete slabs and also providing insightful reference for analyzing the damage power of the produced secondary fragments.