A visible-light induced green and efficient method is developed for the synthesis of α-indolyl diethyl malonates. The reaction proceeds without any photocatalysts or ligands in a green solvent in a short time. Moreover, the reaction mechanism has been clearly studied by control experiments, spectrophotometric studies and density functional theory (DFT) calculations. The results showed that the photocatalyst-free transformation may proceed via an XB-promoted radical process. The EDA complex formation of diethyl bromomalonate with a base is the main reason for the reaction initiation.