The reaction mechanism between propadienylidene and methyleneimine was systematically investigated employing the second-order Moller-Plesset perturbation theory (MP2) method with the 6 - 31 + G* basis set. Geometry optimization, vibrational analysis, and energy property of the involved stationary points on the potential energy surface were calculated. The energies of the different species were corrected by single point energy calculations at the CCSD (T) // MP2 / 6 - 31 + G* level. From the surface energy profile, one important initial intermediate characterized by a 3-membered ring structure was located via a transition state firstly. After that, 3 different products possessing 3- and 4-membered ring characters were obtained through corresponding reaction pathways. In the first reaction pathway (1), a 3-membered ring alkyne compound was obtained. A 4-membered ring conjugated diene compound was produced in the other 2 reaction pathways, pathways (2R) and (2L). The energy barrier of the rate-determining step of pathway (1) is higher than those of the pathways (2R) and (2L), where the ultimate products of pathways (2R) and (2L) are more stable than that of pathway (1). Therefore, the dominating product of the addition reaction between propadienylidene and methyleneimine should be the 4-membered ring conjugated diene compound.