The impact of blast load is always taken into consideration in significant building and protective construction. In order to study the mechanical behavior of the concrete slab under blast load, the dynamic responses of a square concrete slab under surface explosion are simulated by the nonlinear finite element program ANSYS/LS-DYNA. The JOHNSON_HOLMQUIST_CONCRETE model is used for concrete material and the damage and strain rate effect are considered in this mode1. The simplified exponential load is used as blast load. The Z-displacement,Mises effective stress and damage state of concrete slab under different blast load of corresponding scaled distance are investigated and discussed. It can be found from the results that the influence of blast pressure can be neglected when scaled distance is around 2.0 and the damage of slab is beginning from the edge.