Ionic liquids (ILs) have been applied as a promising solvent for the separation of different kinds of compounds. But the enormous combination possibilities of the cations and anions make it a challenging task to screen an appropriate IL for a specific application. Thus, the method for screening IL candidates based on the COSMO-SAC model is presented and illustrated to extract heterocyclic nitrogen compounds, indole and carbazole, from coal tar fractions. Also, the sigma-profile analysis is used to evaluate the effect of different types of cations and anions, and systematically confirmed the structure-activity relationships for the extraction process. Then, 22 cations and 19 anions are screened as the initial structures of the potential extractants from a sigma-profile database (containing 39 cations and 29 anions). Along with the rigorous evaluation criterion proposed in this work, three IL extractants are adopted. Afterwards, the extractive ability of the candidates is experimentally investigated.