A heterojunction photocatalyst based on porous tubular g-C3N4 decorated with CdS nanoparticles was fabricated by a facile hydrothermal co-deposition method. The one-dimensional porous structure of g-C3N4 provides a higher specific surface area, enhanced light absorption, and better separation and transport performance of charge carriers along the longitudinal direction, all of which synergistically contribute to the superior photocatalytic activity observed. The significantly enhanced catalytic efficiency is also a benefit originating from the fast transfer of photogenerated electrons and holes between g-C3N4 and CdS through a built-in electric field, which was confirmed by investigating the morphology, structure, optical properties, electrochemical properties, and photocatalytic activities. Photocatalytic degradation of rhodamine B (RhB) and photocatalytic hydrogen evolution reaction were also carried out to investigate its photocatalytic performance. RhB can be degraded completely within 60 min, and the optimum H2 evolution rate of tubular g-C3N4/CdS composite is as high as 71.6 μmol h−1, which is about 16.3 times higher than that of pure bulk g-C3N4. The as-prepared nanostructure would be suitable for treating environmental pollutants as well as for water splitting.