Novel SnSe/g-C3N4 photocatalysts were one-step synthesized via a microwave heating assisted process in 35 min with SnSe and melamine as precursors. The as-synthesized SnSe/g-C3N4 worked very well in H2 evolution via photocatalysis. Under simulated sunlight, the best SnSe/g-C3N4 sample displayed a H2-production velocity of 1064 μmol g?1 h?1, which is 1.8 folds faster than that of neat g-C3N4. Similar promotion effect was also observed under visible light. To reveal the nature behind the high photoactivity, a thorough investigation was performed. XRD and XPS experiments proved the binary constitution of the composite. DRS experiment demonstrated that the addition of SnSe improved the photoabsorption performance. N2-adsorption analysis showed that the SnSe/g-C3N4 photocatalyst presented similar surface area as g-C3N4. TEM experiments showed that some bulk SnSe were spontaneously decomposed to nanoparticles and finely dispersed in g-C3N4 during the microwave heating process. These SnSe nanoparticles were believed to be the active phase and constructed a heterojunction structure with g-C3N4, resulting in the enhanced charge separation. This conclusion was considered as the key factor leading to the high H2-evolution performance and was further confirmed by the PL, EIS, and PC experiments. The present work provides a feasible and rapid method for the construction of g-C3N4 based photocatalysts.