The effect of strain rate in 1/4 model on blast scaling law was analyzed. The scaling method of simulation explosion test was studied by numerical simulation. Results from the simulation show that similarity of concrete slab subjected to the blast loading can be approximately obtained by using replica scaling law. The mesh size effect on the simulation was investigated. It is found that 4cm meshing results in worse deviations. The errors partly come from the local damage model for the concrete, partly from the coarse meshing.