Abstract: The Monte-Carlo method can resolve the problem that every parameter is often regarded as a constant during the process of calculating coalbed gas resource.This paper mostly describes the main arithmetic and functions realization in calculating coalbed gas resource by Monte-Carlo method,which includes parameter choice,definition of distribution function,generation of false random number and evaluation of the parameter corresponding to false random number.The Monte-Carlo software is programmed by above arithmetic and the software example shows that there is small difference in results between two methods,and Monte-Carlo method has more reliability.