The CoAPSO-5 membrane on the α-Al2O3 substrate was synthesized by using in situ crystallization. According to detection results of scanning electron microscopy (SEM) and X-ray diffraction (XRD), the 20μm thick membrane was composed of well intergrown CoAPSO-5 crystals, which were tightly covered on the α-Al2O3 substrate in random orientation. The gas permeation measurements revealed the resulting membrane was of high-quality with few non-zealitic pores. In addition, the separation property of CO2/CH4 through the synthesized CoAPSO-5 membrane was investigated. When the osmotic pressure was 0.3MPa and 0.5MPa, the real separation factor of CO2/CH4 was 4.86 and 5.29 respectively.