A detailed theoretical investigation for the ion-molecule reaction of HCN+ with C2H2 is performed at the B3LYP/6-311G(d,p) and CCSD(T)/6-311++G(3df,2pd) (single-point) levels. Possible energetically allowed reaction pathways leading to various low-lying dissociation products are probed. It is shown that eight dissociation products P-1(H2C3N++H), P-2(CN+C2H3+), P-3(HC3N++H-2), P-4 (HCCCNH++H), P-5(H2NCCC++H), P-6(HCNCCH++H), P-7(C2H2++HCN), and P-8 (C2H2++HNC) are both thermodynamically and kinetically accessible. Among the eight dissociation products, P-1 is the most abundant. product. P-7 and P-3 are the second and third feasible products but much less competitive than P-1, followed by the almost negligible product P-2. Other products, P-4 (HCCCNH++H), P-5 (HCNCCH++H), P-6 (H2NCCC++H), and P-8 (C2H2++HNC) may become feasible at high temperatures. Because the intermediates and transition states involved in the reaction HCN+ + C2H2 are all lower than the reactant in energy, the title reaction is expected to be rapid, as is consistent with the measured large rate constant at room temperature. The present calculation results may provide a useful guide for understanding the mechanism of HCN+ toward other pi-bonded molecules.