This article presents a concept of critical thickness in low permeable coalbed methane reservoirs with a simulation study when a horizontal well is used. The equations proposed to finish the simulation study are based on the Langmuir equation, Fick pseudo-steady state law, and Darcy equation, in which equivalent radius model and function of pseudo pressure are combined to consider the effect of diffusivity and compressibility. The method of finite difference is presented to solve the equations. Then we established the partial differential equations and the definite conditions that describe the mechanism of coal gas and water flow. As the low permeable property of a coal bed in China, we discussed the applicability of a horizontal well according to the reservoir thickness. The significant results from our study include: (1) There is critical reservoir thickness in developing low permeable coalbed methane reservoir with horizontal well. (2) When the thickness is less than the critical value, the cumulative gas production will be lower and the commercial efficiency is unsatisfactory. When the value is more than the critical one, the productivity will not increase as quickly as the reservoir thickness. On the contrary, it will be almost stable along with the reservoir thickness increase. (3) The production effect of the horizontal well on a low permeable coalbed methane reservoir will be decreased when the reservoir thickness is too high. (4) The peak gas rate will go down first and move upward later as the reservoir thickness increases. Consequently, the adaptability of the horizontal well to a low permeable coalbed methane reservoir should be analyzed first according to the reservoir thickness.