In this paper, CdTe quantum dot (QD)@silica nanobeads were used as modulated photoluminescence (PL) sensors for the sensing of ascorbic acid in aqueous solution for the first time. The sensor was developed based on the different quenching effects of Fe²⁺ and Fe³⁺ on the PL intensity of the CdTe QD@ silica nanobeads. Firstly, the PL intensity of the CdTe QDs was quenched in the presence of Fe³⁺. Although both Fe²⁺ and Fe³⁺ could quench the PL intensity of the CdTe QDs, the quenching efficiency were quite different for Fe²⁺ and Fe³⁺. The PL intensity of the CdTe QD@silica nanobeads can be quenched by about 15% after the addition of Fe³⁺ (60 μmol L⁻¹), while the PL intensity of the CdTe QD@silica nanobeads can be quenched about 49% after the addition of Fe²⁺ (60 μmol L⁻¹). Therefore, the PL intensity of the CdTe QD@silica nanobeads decreased significantly when Fe³⁺ was reduced to Fe²⁺ by ascorbic acid. To confirm the strategy of PL modulation in this sensing system, trace H₂O₂ was introduced to oxidize Fe²⁺ to Fe³⁺. As a result, the PL intensity of the CdTe QD@silica nanobeads was partly recovered. The proposed sensor could be used for ascorbic acid sensing in the concentration range of 3.33–400 μmol L⁻¹, with a detection limit (3σ) of 1.25 μmol L⁻¹ The feasibility of the proposed sensor for ascorbic acid determination in tablet samples was also studied, and satisfactory results were obtained.