Hierarchical CdS nanocrystals were synthesized with chitosan as the additive by solvothermal method. The concentration of chitosan had obvious influence on the morphology of CdS. When 2.5 mg/mL chitosan was added into the system, spherical CdS with the size of 200–300 nm assembled by hexagonal microspheres was formed. CdS was characterized by X-ray diffraction pattern (XRD), transmission electron microscopy (TEM), thermogravimetry (TG), Fourier transform infrared spectroscopy (FT-IR), UV-visible spectroscopy (UV-vis), and cyclic voltammetry (CV). CdS had the wurtzite structure with slight orientation growth along  direction. The chitosan as the additive only participated in the reaction process and would not attach on the surface of final product. Hierarchical CdS displayed good absorption in the visible region, which was applied in the photodegradation to rhodamine B. The photocatalysis displayed the dependence on the morphology of CdS, which was resulted from different electron-hole recombination ability confirmed by fluorescence (PL) spectra. The maximum photodegradation efficiency nearly to 97% and constant rate close to 0.03 s−1 were obtained, which was attributed to the maximum charge separation efficiency. The photodegradation products were measured by liquid mass spectrometry (LC-MS), and the photodegradation mechanism was also discussed.