Graphite phase carbon nitride heterojunction composites were prepared by calcination at 600 ℃ with dicyandiamide and ammonium thiocyanate as precursor. The materials were characterized by X-ray diffractometer (XRD),scanning electron microscope (SEM), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FTIR) and UV-visible diffuse reflectance absorption spectrum (UV-vis DRS). The catalytic activity of g-C3N4 under visible light (λ>420 nm) and the recycling stability were investigated using rhodamine B (RhB) as a simulated pollutant. The degradation mechanism was explored by the active species capture experiment. The results showed that the photocatalytic degradation efficiency of g-C3N4 prepared by the mixture of dicyandiamide and ammonium thiocyanate at a ratio of 3∶2 reached 92. 53% in 2 h, which was 1. 35 times that of g-C3N4 prepared by pure dicyandiamide and 1. 75 times that of g-C3N4 prepared by pure ammonium thiocyanate. The g-C3N4 prepared by dicyandiamide and ammonium thiocyanate as precursor is a homotype heterojunction, which improves the separation efficiency of photogenerated carriers. The active species capture experiment showed that the superoxide radical (·O2- ) plays a major role in catalytic activity.