Medium-sized ring lactams have been found as key structures in various bioactive molecules, and they are of vital importance in natural or pharmaceutical molecule synthesis. In this work9-membered-ring lactams were synthesized with graphitic carbon nitride (g-C3N4) as photocatalyst, BI-OAc as oxidant under visible light in optimized mild conditions. These reactions exhibit a broad scope of substrates with good to excellent yield (up to 90%). In parallel, an amidyl-radical-involved photocatalytic ring expansion mechanism was proposed. This work paves a new way for the synthesis of medium-sized-ring molecules in a green and economical method.