Under the condition of microwave assisted, formaldehyde functionalized polyethyleneimine (FPEI) was synthesized. The maximum emission of FPEI at 470 nm was obtained when the excitation wavelength was set at 340 nm, and the blue fluorescence was easily observed under an ultraviolet lamp. Under this excitation condition, the fluorescence emission peak of the Eosin Y (EY) was located at 540 nm, and the green fluorescence was observed. In acidic medium, the EY and FPEI form a complex through electrostatic interaction. The formation of the FPEI/EY complex resulted in the quenching of EY fluorescence at 540 nm, while the fluorescence intensity of FPEI was only slightly reduced. In the presence of sodium hexametaphosphate (SHMP), competitive binding occurred between SHMP, EY and FPEI. Because the electrostatic interaction between SHMP and protonated amino groups on FPEI surface is stronger than that of EY, EY was gradually released from the PEI/EY complex, resulting in the recovery of EY fluorescence at 540 nm. The fluorescence change ratio between the fluorescence intensity of Ey at 540 nm and the fluorescence intensity of FPEI at 470 (F₅₄₀/F₄₇₀) has a good linear correlated with the concentration of SHMP, and a noticeable blue to green changed under an ultraviolet lamp if SHMP solution was mixed with the FPEI/EY. Based on the phenomena, a novel rapid ratiometric fluorescence method for determination of SHMP was developed. Under the optimized conditions, the linear range is 0.1~4.2 μmol·L^-1, and the limit of detection (3σ) was 38 nmol·L^-1. The proposed method is of high selectivity, simple and rapid, and is successfully applied to determine the concentration of SHMP in tea drink samples.