A new zeolitic imidazolate framework-8 (ZIF-8) composite membrane was prepared by a fast in situ seeding method. The commercial anodic aluminum oxide (AAO) membrane was used as the support, which was immersed in the ZIF-8 precursor solution to allow the solution to diffuse into the AAO nanochannels. Ammonium hydroxide was then added to quickly nucleate ZIF-8 nanocrystals in the AAO channels as seeds. After secondary growth at room temperature, the ZIF-8/AAO composite membrane was obtained with ZIF-8 nanocrystals plugged in the AAO nanochannels, and the membrane exhibited high gas separation performance with H2/CO2 and H2/N2 ideal selectivities of 6.38 and 4.19, respectively. For comparison, only a layer of ZIF-8 was formed on the AAO membrane surface by a normal dip-coating method with ammonium hydroxide pre-added in the precursor solution, and the resulting membrane was not dense and continuous.