The invention discloses a pH detecting method based on copper-zinc-tin-sulfur alloy quantum dots. The copper-zinc-tin-sulfur alloy quantum dots use low-toxic metal salt compounds of copper salt, zincsalt, and stannous salts, and high molecular weight polymer as a coating stabilizer. The copper-zinc-tin-sulfur alloy quantum dots are synthesized under hydrothermal conditions by dropwise adding thiourea, and increase with the proportion of the coating agent. The emission wavelength can be gradually shifted from the ultraviolet light region to the blue light region and the visible light region. In the pH detection, the copper-zinc-tin-sulfur alloy quantum dots are used as fluorescent labels to quench the fluorescence intensity of the detection system. That is, the pH and the fluorescence intensity exhibit a linear quenching relationship, and the pH of the system to be tested can be obtained by comparing the standard curve.