The significant feature of coalbed in China is the low permeability. A new unsteady seepage flow model is developed for the low permeability coalbed by considering the startup pressure gradient and methane desorption effect. Since the complexity of the problem, a new method which we call it "L-FVM" is developed, based on comparing the normal numerical calculation methods and comprehension research on FVM. The results show that L-FVM has the same precission but higher calculating velocity than normal FVM. This result is very important for monitoring the area pressure drawdown in coalbed methane engineering.