We have developed a novel fluorescence sensor based on upconversion nanoparticles (UCNPs) for the rapid and sensitive detection of trace aluminum ions (Al3+). The sensor utilizes the inner filter effect (IFE) between the UCNPs and the xylenol orange-aluminum complex (XO-Al3+), resulting in significant fluorescence quenching at 543 nm upon Al3+ binding. This quenching correlates directly with the Al3+ concentration, allowing for quantitative detection within a range of 0-30 μM and achieving an ultra-low detection limit of 0.19 μM. Selectivity of the sensor is enhanced by the incorporation of ascorbic acid, which masks interfering Fe3+ ions, thus ensuring accurate determination of Al3+ even in the presence of other metal ions. The UCNPs-XO sensor exhibits excellent stability and reproducibility, and minimal interference from commonly co-existing substances. This makes it suitable for the detection of Al3+ in various matrices, including food products and environmental water samples. Our work offers a significant advancement in Al3+ detection, with potential applications in food safety, environmental monitoring, and public health.