The intramolecular [2+2] cycloaddition of 1,3-dienes under visible light irradiation investigated by Yoon and his co-workers shows remarkably high yield and stereoselective differences under different photocatalysts. The reaction was speculated to be induced by energy transfer. However, the origin for these phenomena is still unclear. In this scene, the detailed mechanism for the [2+2] cycloaddition of 1,3-dienes under visible light has been investigated using density functional theory B3LYP and TPSSTPSS methods. The result shows that the reaction not only can be induced by energy transfer between photocatalysts and reactants, but also can be induced by electron transfer between them. The [2+2] cycloaddition induced by energy transfer is carried out along the potential energy surface (PES) of triplet excited states (T1) firstly, and then goes back to the singlet ground state (S0) via MECPs (minimum energy crossing points) between the PESs of the S0 and T1 states, forming the product in the S0 state. The [2+2] reaction induced by electron transfer proceeds along the doublet state PES of the cation radical reactant and the neutral four-membered ring product could be obtained by electron transfer from the corresponding reactant or reduced photocatalyst. The origin of stereoselectivity of the [2+2] reaction is attributed to the reaction mechanism difference under different photocatalysts.