Secnidazole, a second-generation nitroimidazole, is widely utilized for its high efficacy in treating infections such as bacterial vaginosis, amoebiasis, and giardiasis, especially in the treatment of genital infections in females. Herein, a smartphone-based sensing platform with a UV lamp has been developed, in combination with a microfluidic-based paper device, to enable a visual, quick, instrument-free, sensitive, and quantitative analysis in real-time conditions. Through a fluorescence "turn-off" response, the platform demonstrated the feasibility of visual quantitative drug detection. In addition to having a high sensitivity for the label-free and on-site detection of secnidazole, our suggested smartphone-assisted N-CDs@WS₂ composite demonstrated a prominent anti-interference capability to ensure accurate point-of-care identification of secnidazole based on the fluorescent output, successfully removing the effects of environmental fluctuations. The N-CDs@WS₂-based fluorescent sensor has a limit of detection (LOD) of 0.117 µM and 117 nM for secnidazole, while its limit of quantification (LOQ) is 0.3573 µM and 357 nM. The fact that such a portable sensor has effectively achieved real-time monitoring of secnidazole in real blood serum samples with encouraging outcomes opens up a promising path to accurate on-site determination. The detection efficiency of the suggested fluorescent sensor was confirmed through an LC-MS analysis and the UPLC method, which is recommended by the US Environmental Protection Agency. As indicated by the recoveries from the UPLC and LC-MS analyses and the fluorescence of N-CDs@WS2, the current sensor is capable of quantitatively analyzing secnidazole in real samples.