Abstract: When detonation gas pressure acts on crack surface of an oil well,the surrounding rock is subjected to dynamic response,and gas pressure varies with fracturing process,then the fracturing process is influenced by gas pressure variation.Special crack element was used in the proposed model to simulate the opening and closing process of crack.Such a dynamic FEM equation involving contact problem was solved by Newmark method,and the proposed gas distribution function was also considered.The numerical result shows: ①detonation gas had a major role in the fracturing rock comparing with elastic stress wave;②crack growth velocity was smaller as the initial tectonic stress was greater;③crack growth velocity was lager as the initial crack length was longer;④crack initiated relatively later as pressure rose fast,which meant a slowly rising gas pressure would produce a longer crack.