In this work, the ultrasound-assisted hydrothermal synthesis method offers a facile method to synthesize highly efficient photoluminescence sulfur quantum dots (SQDs). Impressively, a switchable fluorescent "on-off-on" sensor was developed using the acquired SQDs, which are capable of sequentially detecting iron ions (Fe3+) and ascorbic acid (AA) with exceptional sensitivity and selectivity. Meanwhile, SQDs and Fe3+ formed complexes through coordination, causing the fluorescence quenching of SQDs because of the static quenching effect. Upon the addition of AA into the SQDs/Fe3+ system, a redox-reaction-mediated mechanism leads to the recovery of fluorescence. The fluorescence intensity of SQDs exhibits a linear relationship with the concentrations of Fe3+ and AA in the ranges 5-30 and 20-100 μM, respectively. Notably, the detection limits achieved are 14.31 nM for Fe3+ and 0.64 μM for AA. Moreover, the chemosensor was successfully employed for monitoring Fe3+ in real water samples and AA in fruits. These results demonstrate the excellent analysis and detection capabilities of SQDs in the complex environment.