The mechanism of the cycloaddition reaction of forming a silicic bis-heterocyclic compound between singlet dimethylmethylenesilylene （Me2C=Si：） and ethene has been investigated with the CCSD（T）//MP2/6-31G＊ method. From the potential energy profile, it can be predicted that, this reaction has one dominant channel. The presented rule of this dominant channel： the 3p unoccupied orbital of Si in dimethylmethylenesilylene and the π orbital of ethene forming the π→p donor-acceptor bond, resulting in the formation of three-membered ring intermediate （INT1）; INT1 then isomerizes to a four-membered ring silylene （P2）, which is driven by ring-enlargement effect; due to sp3 hybridization of Si atom in P2, P2 further combines with ethene to form a silicic bis-heterocyclic compound.
