B-doped g-C3N4 was prepared in the laboratory via heating a mixture of melamine and boric acid. The synthesized material was characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) analysis, which revealed the high specific surface area and large amount of active groups on the surface of B-doped g-C3N4. The sorption of cadmium from aqueous solutions by B-doped g-C3N4 was studied under equilibrium conditions in the concentration range of 0.01 similar to 5.0 mmol/L. The pH of the solution was varied over a range of 2 similar to 6. The sorption of cadmium on the material was determined to be pH-dependent, and the Lagergren-second-order kinetic model was suitable to simulate the sorption process. The maximum sorption capacity from the Langmuir model was determined to be 1.4162 mmol/g (about 159.2 mgCd/g). XPS and FTIR data suggest that cadmium ions were mainly attached to the N-H and O-H groups on the surface of B-doped g-C3N4.