Organophosphorus pesticide residues pose significant risks to human health, but conventional analytical methods are costly and complex. To address this, we developed a new environmentally friendly technique using a simple hydrothermal method to create a ratiometric fluorescent composite (RFC) with two distinct signals: blue and orange. This RFC allowed for rapid detection and quantitative analysis of organophosphorus pesticides. The composite incorporated a cerium-based metal–organic framework (Ce-UiO-66), which could degrade methyl paraoxon (MP) due to the two oxidation states of cerium. The resulting degradation product, p-nitrophenol (p-NP), specifically quenched the blue fluorescence of the RFC via the inner filter effect (IFE), while the orange fluorescence remained unaffected, ensuring consistency under microenvironmental and spectrometer conditions. This innovative approach was sensitive, with a detection limit of 0.55 μM, selective, and repeatable, offering a new strategy for pesticide analysis and safety evaluation of edible and medicinal plant and water samples.