Gas-liquid-solid (G-L-S) three-phase slug flow provides an efficient pathway to utilize solid catalysts in continuous flow and was adopted in the mesoporous graphite carbon nitride (mpg-C3N4)-catalyzed photoinduced electron/energy transfer reversible addition-fragmentation chain transfer (PET-RAFT) polymerization of methyl methacrylate (MMA) in this work. Kinetic studies and chain extension experiments illustrated the realization of reversible deactivation radical polymerization (RDRP) and the "easy to scale up"advantage of a continuous-flow reactor as compared to its batch counterpart. The light intensity played an important role on the PET-RAFT polymerization. An increasing amount of photocatalyst favored the monomer conversion within a limited range due to higher light blockage, and the monomer conversion reached a stable level at a lower catalyst concentration when higher light power was applied. When compared with fully continuous flow, the G-L-S slug flow was beneficial to the PET-RAFT polymerization due to the intensified swirling strength and narrower velocity field. Decreasing the gas-To-slurry ratio also led to narrower velocity distribution, which favored the polymerization as well. Moreover, the polymerization rates remained stable in multiple recycles, demonstrating that the present G-L-S slug flow was a reliable and easy processing approach for utilizing the solid catalyst.