The room-temperature ionic liquids (RTILs) have potential in realizing the ethylene (C2H4) and acetylene (C2H2) separation and avoiding solvent loss and environmental pollution compared with traditional solvents. The interaction mechanisms between gases and RTILs are important for the exploration of new RTILs for gas separation; thus, they were studied by quantum chemical calculation and molecular dynamics simulation in this work. The optimized geometries were obtained for the complexes of C2H4/C2H2 with anions (Tf2N-, BF4-, and OAc-), cation (bmim(+)), and their ion pairs, and the analysis for geometry, interaction energy, natural bond orbital (NBO), and atoms in molecules (AIM) was performed. The quantum chemical calculation results show that the hydrogen-bonding interaction between the gas molecule and anion is the dominant factor in determining the solubility of C2H2 in RTILs. However, the hydrogen-bonding interaction, the p-pi interaction in C2H4-anion, and the pi-pi interaction in C2H4-cation are weak and comparable, which all affect the solubility of C2H4 in RTILs with comparable contribution. The calculated results for the distance of H-gas center dot center dot center dot X (X = O or F in anions), the BSSE-corrected interaction energy, the electron density of H-gas center dot center dot center dot X at the bond critical point (rho(BCP)), and the relative second-order perturbation stabilization energy (E(2)) are consistent with the experimental data that C2H2 is more soluble than C2H4 in the same RTILs and the solubility of C2H4 in RTILs has the following order: [bmim] [Tf2N] > [bmim][OAc] > [bmim] [BF4]. The calculated results also agree with the order of C2H2 solubility in different RTILs that [bmim][OAc] > [bmim][BF4] > [bmim][Tf2N]. Furthermore, the calculation results indicate that there is strong C2H2-RTIL interaction, which cannot be negligible compared to the RTIL-RTIL interaction; thus, the regular solution theory is probably not suitable to correlate C2H2 solubility in RTILs. The molecular dynamics simulation results show that the hydrogen bond between the H in C2 of the imidazolium cation and the anion will weaken the hydrogen-bonding interaction of the gas molecule and anion in a realistic solution condition, especially in the C2H4-RTIL system.