The potassium and iodine co-doping combined with a mesoporous structure for developing a novel g-C3N4-based photocatalyst is constructed. The K and I co-doped mesoporous g-C3N4 has been successfully synthesized via the one pot thermal polymerization of a mixture consisting of dicyandiamide, KI, and SBA-15 used as the hard template. It is shown that this photocatalyst consists of short rod-like network with plenty of pores openings into their surfaces. The mesoporous structure and the doping with K and I in it have been verified by several techniques. For this photocatalyst, a blue shift of the optical absorption band edge induced by the mesoporous structure is compensated for by the red shift originated from the co-doping with K and I. This photocatalyst exhibits the longest life time of carries, the fastest charge transport, and the highest photocurrent density as compared with the mesoporous g-C3N4 and the K and I co-doped one. It is revealed a synergistic effect between the formation of the mesoporous structure and the co-doping with K and I. The synergistic effect make the K and I co-doped mesoporous g-C3N4 photocatalyst achieves a high hydrogen evolution rate of 80.58 mu mol/h, 9.7 times as high as that for pristine g-C3N4. These results may shed light on the integration of different modification strategies for developing novel g-C3N4-based photocatalysts with much enhanced photocatalytic activity.