Herein, a fluorescence "turn ON-OFF" switch model PB@EuW10/SiO2 core shell composite is designed and fabricated by coating EuW10-containing silica layer on Prussian blue (PB) nanoparticles via a facile method. It is found that the presence of PB can quench the photoluminescence of the composite which arises from EuW10. When hydrazine is mixed with the composite dispersion, PB can be reduced to Prussian white (PW), resulting in the decrease of absorption and the appearance of photoluminescence (turn ON). In addition, PW can also be converted back to PB, which is achieved by adding hydrogen peroxide, and the photoluminescence of the composite is quenched again (turn OFF). Thus, the composite is applied for N2H4 and H2O2 detection by fluorescence spectroscopy and UV-vis absorption spectroscopy. Wide linear ranges for N2H4 and H2O2 detection with low detection limits are found for both detection methods on the PB@EuW10/SiO2 core@shell composite. Besides, the color from light blue to colorless of the detection dispersion can also indicate the turn ON-OFF switch for fluorescence. Furthermore, the proposed model can also be extended to other composites.