A visible-light metal-free photocatalytic synthesis of 3-position substituted 3,4-dihydroisoquinolin-1(2H)-one derivatives under mild conditions in moderate to good yields is described. EosinY Na, an organic dye, which is of low cost and has good availability, is used as the photocatalyst. A wide range of substrates are tolerated and the gram-scale reaction can also proceed smoothly. Mechanistic studies indicate that a plausible free radical process is proposed.