Olefin/paraffin separation is critical for modern chemical industry, yet consumes huge amount of energy as these molecules are very similar. ZIF-8 polymorphs brought very promising opportunities to solve such problem by showing unprecedented propane/propylene separation performance in our recent work (S. Zhou, Y. Wei, L. Li, Y. Duan, et al., Sci. Adv., 2018, 4, eaau1393), whereas some relevant scientific questions remain poorly understood: 1. How the geometrical difference among these polymorphs lead to the superior gas separation performance? 2. How to develop polymorphs with even better adsorption/separation performance? Herein, Grand Canonical Monte Carlo (GCMC) simulations and the IAST model were employed to study the adsorption of propane/propylene on several ZIF-8 polymorphs with a large variety of topology, which is critical for understanding and implementing ZIF-based adsorption or membrane separation process. Our simulation results well agree with the experimental data, revealing the adsorption and separation mechanism of olefin/paraffin on the ZIF-8 polymorphs. The ZIFs’ interactions with gas and their porosity were found to well correlate with the gas uptake, and the density distribution contours further revealed the adsorption competition between propane and propylene. The adsorption of propane + propylene mixtures on ZIF-8 polymorphs was further studied via multi-component GCMC and IAST, both yielding similar results, such as propane selectivities and uptakes. This work not only provided with in-depth insights to understand the adsorption and separation of olefin/paraffin on ZIFs, but also validated combining GCMC with IAST as an efficient methodology to study this subject.