Mechanism of the cycloadditional formaldehyde has been investigated reaction between singlet with MP2/6-31G^＊ method, dichloro-germylidene and including geometry optimization, vibrational analysis and energies for the involved stationary points on the potential energy surface. From the potential energy profile, we predict that the cyeloaddition reaction between singlet dichloro-germylidene and formaldehyde has two competitive dominant reaction pathways, going with the formation of two side products （INT3 and INT4）, simultaneously. Both of the two competitive reactions consist of two steps, two reactants firstly form a three-membered ring intermediate INT1 and a twisted four-membered ring intermediate INT2, respectively, both of which are barrier-free exothermic reactions of 41.5 and 72.3 kJ/mol; then INT1 isomerizes to a four-membered ring product P1 via transition state TS1, and INT2 isomerizes to a chlorine-traasfer product P2 via transition state TS2, with the barriers of 2.9 and 0.3 kJ/mol, respectively. Simultaneously, P1 and INT2 further react with formaldehyde to form INT3 and INT4, respectively, which are also barrier-free exothermic reaction of 74.9 and 88.1 kJ/mol.
