Abstract: H₂Ge=Si: and its derivatives (X₂Ge=Si:, X=H, Me, F, Cl, Br, Ph, Ar,…) are new species. Its cycloaddition reactions are new area for the study of silylene chemistry. The cycloaddition reaction mechanism of singlet H₂Ge=Si: and formaldehyde has been investigated with the MP2/aug-cc-pVDZ method. From the potential energy profile, it could be predicted that the reaction has one dominant reaction pathway. The reaction rule is that two reactants firstly form a four-membered Ge-heterocyclic ring silylene through the 2+2 cycloaddition reaction. Because of the 3p unoccupied orbital of Si: atom in the four-membered Ge-heterocyclic ring silylene and the π orbital of formaldehyde forming a π→p donor-acceptor bond, the four-membered Ge-heterocyclic ring silylene further combines with formaldehyde to form an intermediate. Because the Si: atom in the intermediate undergoes sp³ hybridization after transition state, then the intermediate isomerizes to a spiro-Si-heterocyclic ring compound involving Ge via a transition state. The result indicates the laws of cycloaddition reaction between H₂Ge=Si: or its derivatives (X₂Ge=Si:, X=H, Me, F, Cl, Br, Ph, Ar,…) and asymmetric π-bonded compounds are significant for the synthesis of small-ring involving Si and Ge and spiro-Si-heterocyclic ring compounds involving Ge.