In natural water, phosphorus is an important indicator for evaluating water quality, which could cause excessive growth of algae. In this paper, graphene quantum dots (GGQDs in short) with a maximum fluorescence emission wavelength of 424 nm was synthesized by using glutathione and citric acid as a raw material, which has been mixed with CdTe quantum dots (maximum fluorescence emission wavelength of 578 nm) to form a ratio fluorescence detection system (GCS in short). Based on the linear relationship between the change of fluorescence intensity (F 424 nm / F 578 nm) and the concentrations of phosphate, a quantitative analysis method for phosphate has been established. The linear range of this method is 15 - 90 μmol/L, the lowest detection limit is 6.66 μmol/L. This method has high sensitivity, selectivity and stability, could be used for detection of phosphate water.