Abstract: Water soluble carbon quantum dots (CND) emitting green photoluminescence were synthesized by hydrothermal carbonization method (220℃) using larch as carbon source and deionized water as solvent. The particle size distribution, structure and optical properties of CND were characterized by transmission electron microscope(TEM), Fourier transform infrared spectroscopy(FT-IR), X-ray photoelectron spectroscopy(XPS), ultraviolet spectrophotometer and fluorescence spectrophotometer. The results indicated that the obtained CND particles were uniform spherical with diameter of 20.35 nm and ordered lattice on surface. CND mainly contained C, O, N elements with -OH, C=O and C-O functional groups on the surface and the CNDs were formed with oxygen-containing group and aromatic ring structure. CND samples had a weaken absorption at 271 nm in the ultraviolet absorption spectrum and exhibited excitation wavelength-dependent fluorescence with the maximum emission and excitation at 350 and 440 nm. And the photoluminescence intensity was pH-dependent. The fluorescence intensity reduced and solution color became darker with the increasing of pH value of CND solution. TiO2/CND composite exhibited high photocatalytic activity for the degradation of tetracycline, and 93.6% degradation could be achieved after 3 h.