The H2Ge=Ge：, as well as and its derivatives （X2Ge=Ge：, X=H, Me, F, C1, Br, Ph, At, ...） is a kind of new species. Its cycloaddition reactions is a new area for the study of germylene chemistry. The mechanism of the cycloaddition reaction between singlet Me2Ge=Ge： and acetaldehyde was investigated with the B3LYP/6-31G＊ method in this work. From the potential energy profile, it could be predicted that the reaction has one dominant reaction pathway. The reaction rule is that the two reactants firstly form a four-membered Ge-heterocyclic ring germylene through the [2＋2] cycloaddition reaction. Because of the 4p unoccupied orbital of Ge： atom in the four-membered Ge-heterocyclic ring germylene and the ~ orbital of acetaldehyde forming a r^--~p donor-acceptor bond, the four-membered Ge-heterocyclic ring germylene further combines with acetaldehyde to form an intermedi- ate. Because the Ge atom in intermediate happens sp3 hybridization after transition state, then, intermediate isomerizes to a spiro-Ge-heterocyclic ring compound via a transition state. The research result indicates the laws of cycloaddition reaction between Me2Ge=Ge： and ac- etaldehyde, and lays the theory foundation of the cycloaddition reaction between H2Ge=Ge： and its derivatives （X2Ge=Ge：, X=H, Me, F, C1, Br, Ph, At, ...） and asymmetric ^-bonded compounds, which are significant for the synthesis of small-ring and spiro-Ge-heterocyclic ring compounds.
