Abstract：On the basis of theoretical model of hull girder’s plasticity deformation due to impact from blast, variables that affect dynamic response of the girder were defined. principle was used to analyze the similarity law between model and prototype, and accurate similarity parameters and conditions were proposed. Inaccurate similarity problem caused by strain-rate effect was discussed considering effects of model scale, strain-rate and materials on similarity. Results show that the smaller scale or the larger strain-rate, the larger the error of prediction is. If structural material is more sensitive to strain-rate effect, the error will be larger too. The conclusions obtained are useful to guide scaling experiments in which hull girder model subjected to shock wave is to predict dynamic response of a warship.