Software FLACS was used for 3D modeling and 3D dynamic simulation of effects of different ignition po- sitions，different equivalence ratio of fuel-air and the number of stacking layers on explosion overpressure，in order to study the explosion characteristics of pentane vapor cloud in a class A warehouse． And the worst explosion sce- nario was determined． Taking the explosion of pentane vapor cloud as an example，the simulation results show that when the ignition position is at the edge of the pentane vapor cloud and closed to the firewall，large explosion over- pressure is easily produced; the maximum explosion overpressure increases first and then decreases with increase of the equivalent ratio of pentane-air，and decreases with the decrease of the number of stacking layer; under the worst explosion scenario (ignition position is in the middle of the firewall，number of stacking layer is 3 and equiva- lent ratio of pentane-air is 1. 35)，the maximum explosion overpressure is 727 kPa． The research results can pro- vide reference for explosion-proof design of class A warehouses for hazardous chemicals．