A nitrogen expansion liquefaction process which is suitable for small-scale liquefaction plants is proposed for coalbed methane (CBM) with high nitrogen content. In order to investigate the effects of the nitrogen content on the liquefaction system performance, optimizations are done for CBM with nitrogen content among 0~70% via simulation and calculation with HYSYS when the liquefaction rate and the methane recovery rate are fixed respectively, and with the unit product liquefaction power consumption as the major index, the corresponding system performance is compared. The results show that, the system performance of CBM liquefaction process changes greatly along with nitrogen content. However, the changing is not monotonous.