Series of Drop Weight Tests with varying sample mass of RDX, tetryl, and a PBX molding powder were carried out and the dramatic effects of sample mass on the explosion probability were obtained. The experimental results indicate that the explosion probability will firstly increase and subsequently decrease when the mass of sample reduces and also it shows the explosion probability is also related with the coverage area of the sample on the impact pillar. The explosion probability reached the maximum when the impact pillar was fully covered by the sample. A Dual-factor-competition Model is proposed in this work, which is able to predict the correlation between the explosion probability and sample mass. The response characteristic obtained can help to redesign the sensitivity test and provide a new way to distinguish the impact sensitiveness of insensitive explosives by reducing the sample mass.