Compared with the specialized membrane applied to separate one type of mixture, the membranes that are able to selectively separate diverse alkane/alkene mixtures are more valuable in chemical and petroleum industry. Herein, we explored nine nanoporous graphene (NG) membranes including three basic pores and six expanded pores, which exhibit excellent permeance and ultrahigh selectivity for C2H4, C3H6 and C-4 H-6 over the corresponding alkanes. From the thermodynamic and dynamic insights, the local size sieving and competitive adsorption mechanisms play the crucial role in capturing alkenes but rejecting alkanes. The flat alkenyl blocks are smaller than the alkyl blocks, which is beneficial for alkenes to access and cross the confined pores. The above conclusions are further applicable for multilayer NG membranes with the one-dimensional channel. We anticipate our results of expanding pores with higher permeability while maintaining the selectivity can provide a valuable guidance for experimenters to fabricate effective nanopores with a certain tolerance in the future.