Coalbed methane (CBM) is an important energy resource in the world, and to recover this important energy, liquefaction is a good option. Different from ordinary natural gas, CBM usually consists of a lot of nitrogen, which cannot be removed by the ordinary purification technology of LNG. One way of separating nitrogen from CBM is by distillation after liquefaction. In this way, nitrogen is liquefied together with methane, so the liquefaction process and its system performance may be different from that of the ordinary natural gas and will change along with the nitrogen content of CBM feed gas. The liquefaction process adopting a mixed refrigerant cycle with propane pre-cooling is discussed in this paper, which is widely used in LNG liquefaction plants. Taking the unit product liquefaction power consumption as the major index for analysis, the optimum parameters of the liquefaction processes at different nitrogen content of CBM feed gas are worked out, and the corresponding system performance is obtained and compared.