Graphite carbon nitride (g-C3N4) as a novel photocatalyst has attracted growing attention, but its photocatalytic efficiency should be further improved. Based on the large work function and fast electron conductivity of carbon nanotubes (CNTs), here CNT/g-C3N4 photocatalysts with improved H-2 evolution ability and stable water splitting ability were synthesized. The improvement was attributed to the synergistic effect between CNTs and g-C3N4. As for the mechanisms, CNTs strongly attracted photoelectrons and, because of excellent conductibility, rapidly transferred photoelectrons from the catalyst interface. Thereby, the photoelectron migration rate and the photogenerated charge separation and the use efficiency of photoelectrons in g-C3N4 were improved, which largely enhanced the hydrogen production ability. Moreover, the addition of CNTs improved the service life and stability of g-C3N4-based photocatalytic H-2 production. After 10hours of visible light irradiation, the maximum H-2 yield from the 12-mg/L CNT/g-C3N4 (CG12) was 138.7 times larger than that of g-C3N4 (6548.4 vs 47.2mol/g), and the H-2 evolution rate was 138.7 times that of g-C3N4 (654.8 vs 4.72mol/g/h). After 50hours, the apparent quantum efficiency of CG12 was up to 37.9%, indicating that the addition of CNTs improved the photocatalytic splitting and stability of g-C3N4. The mechanism of photocatalytic hydrogen production and the roles of CNTs in improving water splitting were discussed through characterization and activity experiments. It was found that the addition of CNTs accelerated the migration, separation, and utilization of photoelectrons and thereby significantly enhanced the photocatalytic performance.