The presence of illegal additives in weight-loss-promoting foods presents severe risks to consumer health and can even lead to fatal outcomes. To provide robust technical support for addressing this issue, a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed for the simultaneous determination of 42 illegal additives in weight-losspromoting foods. Samples (including tablets, hard capsules, soft capsules, oral liquids and semi-solid preparations) were extracted with methanol via ultrasonication (15 min, 37 kHz), before being diluted with a 30% methanol solution. Each sample was separated by gradient elution on a ZORBAX Eclipse XDB-C18 column (4.6 mm×150 mm, 5 μm) using 0.15% formic acid in methanol as the mobile phase. Detection was performed via LC-MS/MS using an electrospray ionization source, simultaneous positive and negative ion modes, and multiple reaction monitoring mode, with quantification using the external standard method. The results showed that there were good linear relationships (correlation coefficients > 0.995). The limits of quantification (based on a signal-to-noise ratio>10) ranged from 0.5 to 2.0 mg•kg-1, with average recoveries of 70.1% to 117.8% and relative standard deviations<15.0%. Overall, this method is simple and rapid, serving as a reliable analytical tool for the determination of residual illegal additives in weight-loss-promoting foods.