During the production of coal bed methane（CBM），the CBM first is desorbed from coal matrix after the reservoir pressure is dropped by pumping water，and then seepages into the wellbore from the cleat to be produced. According to the production characters of CBM，especially considering the gas diffusion，the compression effect of coal cleat and the shrinkage effect of matrix during the stage of development，a mathematics expression for a CBM material balance model was established based on CBM volumetric balance equation by which an important relation function between CBM reservoir pressure and time was set up to find the change rules of CBM production over time. The solution was achieved through programming to predict dynamic production of CBM. The calculation was done based on a case study of production wells drilled at Yanchang oil and gas field，and the results obtained show that：diffusion of CBM has a remarkable influence on CBM development，and the stronger the diffusion is，the higher the production peak is and the earlier the peak appears. CBM diffusion has a critical role to help stabilize production at later period.