Abstract: A mathematical model for accidental gas release in long transmission pipeline was given,on the basis of computational fluid mechanics.It is found that the existing model for describing hole on a pipe is suitable for predicting gas release through a small punctured hole,while the other existing pipe model is suitable to predict the gas release through a complete break of a pipe.In this paper,a new model for the hole diameter that lies between the above two situations was proposed.The calculation results of an example show that when the initial pressure inside the pipe is higher than 1.5 MPa,the mass of gas released during sonic flow is more than 90 percent of the total mass of the released gas.The total average rate of released gas could be substituted by 30 percent of initial released rate.This approximation would become more accurate with increasing the pressure inside the pipe.