Calcium-based compounds are major inorganic components in coal and also widely used as additives in thermal conversion of coal and, thus, have important effects on the decomposition of nitrogen-containing compounds to form NOx. In this work, the influence of Ca2+ on the pyrolysis of pyrrole and benzopyrrole (indole) to form HCN, a precursor of NOx, was investigated via a density functional theory (DFT) calculation with B3LYP/6-31+G(d,p) basis set. The results suggest that Ca2+ has strong interactions with pyrrole and indole by altering the original electron density distribution of the pyrrole ring and the configurations of pyrrole derivatives, respectively. Ca2+ affects the energy barriers of the elementary pyrolytic reactions (i.e., internal hydrogen transfer, isomerization, and concerted decomposition) and particularly reduces the energy barriers of the rate-determining steps for HCN formation. In comparison to alkali metal ion Na+, alkali earth metal ion Ca2+ has a stronger influence on the pyrrole and indole pyrolysis to form HCN.