•Four allyl imidazolium-based ionic liquids are used as green inhibitors for copper corrosion in sulfuric acid.•The inhibition efficiencies of the investigated inhibitors increase with the augment of alkyl chain length attached to the imidazolium ring.•The order of inhibition ability obtained from EIS is in perfect agreement with the polarization results.•Theoretical calculations provide favorable support for the experimental data. In this work, four allyl imidazolium-based ionic liquids (ILs) with various length of alkyl chains were developed for the corrosion inhibition of copper in 0.5M sulfuric acid. Electrochemical techniques, morphological characterization, theoretical caculations were combined to investigate the inhibition performance and mechanism. The results show that corrosion circumstance of copper in sulfuric aqueous solution is efficiently inhibited by these ILs. Their favorable performance is ascribed to formation of ILs-adsorption films, which follow the Langmuir adsorption isotherm. Besides, the inhibition efficiencies of the investigated inhibitors increase with the augment of alkyl chain length attached to the imidazolium ring.