A detailed theoretical survey on the potential energy surface for the CN + C4H2 reaction is carried out at the QCISD(T)/6-311+G(d,p)//B3LYP/6-311+G(d,p) level. The geometries, vibrational frequencies, and energies of all stationary points involved in the reaction are calculated at the B3LYP/6-311+G(d,p) level. The results suggest that there are two different mechanisms and six pathways in the title reaction. Six products are yielded, and P3: H + HC5N is the dominant product from the view of energy.