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Quantum dots (QDs) are a kind of excellent nano fluorescent materials with complex optical properties. They are simultaneous photon absorber, emitter and scatterer, making it difficult to comprehensively and accurately understand their optical behaviors using conventional methods. Existing studies almost concentrated on their photon absorption and fluorescence features while few studies focused on their light scattering, on-resonance fluorescence (ORF), and so on. In this work, we report a study of the complex optical activities of CdTe QDs using a polarized resonance synchronous spectroscopy (PRS2), combined with Ultraviolet-visible (UV-vis) spectroscopy and fluorescence polarization technologies. This work reveals the complicated optical behaviors of CdTe QDs under the conditions of different ligand functionalization, concentration, incubation time, solvent polarity and provides information including scattering, ORF, UV-vis absorbance spectra, UV-vis extinction, absorption, and scattering cross-section spectra, Stokes-shifted fluorescence (SSF) depolarization and scattering to extinction ratio (SER), etc. Furthermore, we explored the mechanism of QDs as a fluorescent sensor to detect trace Hg2+ from the perspective of light scattering. The observation suggested that the binding of Hg2+ to the QDs leads to their aggregation, which might also contribute to the fluorescence quenching of QDs. Our work is critical for developing a comprehensive understanding of the optical properties of QDs and provides useful information for their actual applications.