To calculate the blasthole pressure in the numerical simulation of blasting,a simplified calculation model was propsed by analyzing the explosive detonation process and the dynamic fracture process of blasted medium.In this model,the blasting process is divided into four stages:the pressure rising,the initial elastic expansion,the crack opening and gas inflow,and the medium stripping and throwing and gas escaping.By analyzing the blasting process,the approximate duration of each stage is determined respectively.By analyzing the change of explosive gas volume and based on the state equation of ideal gas,the time history function of blasthole pressure in each stage is determined.The calculation model is verified by field experiment results and the calculated results are in good agreement with the experimental results.The model takes into account the influence of many factors on the blasting process,,but without the coefficient which is difficult to determine.It can be used to determine the blasting load in theoretical analysis or numerical simulation.