Alkaloids represent a class of naturally-occurring nitrogen-containing compounds widely distributed across diverse plant species. Owing to their well-documented potential to induce adverse effects on human health， certain alkaloids are explicitly prohibited from being used in cosmetic formulations. The escalating global popularity of essential oil-based cosmetics， which commonly incorporate complex botanical extracts， presents a potential avenue for the inadvertent introduction of these prohibited substances. Consequently， the development of robust， sensitive， and efficient analytical methods for their monitoring is of utmost significance for consumer safety and regulatory compliance. This study devises a reliable， high-throughput approach for the simultaneous determination of 13 prohibited alkaloids in essential oil-based cosmetics. It integrates optimized QuEChERS sample preparation with ultra performance liquid chromatography-tandem mass spectrometry （UPLC-MS/MS）. The sample preparation procedure was meticulously designed to maximize efficiency and minimize analyte loss. Chromatographic separation was accomplished on a Waters ACQUITY UPLC HSS T3 column （100 mm × 2.1 mm， 1.8 μm） maintained at a constant temperature of 30 ℃. The mobile phase was composed of （A） acetonitrile and （B） 0.1% （volume fraction） formic acid aqueous solution. A gradient elution program was implemented at a stable flow rate of 0.3 mL/min according to the following profile： an initial 5， a linear increase to 15 （0-2 min）， a rapid rise to 70 （2-4.5 min）， followed by an immediate reversion to the initial 5 （4.5-5.5 min）， which was maintained for re-equilibration until 7.0 min. The injection volume was 5 µL. Detection and quantification were conducted using a triple quadrupole mass spectrometer equipped with an electrospray ionization （ESI） source operating in positive ion mode （ESI⁺）. Data acquisition was carried out in the multiple reaction monitoring （MRM） mode to ensure superior specificity and sensitivity. For each of the 13 alkaloids， two specific ion transitions were monitored： one for quantitative analysis and the other for confirmatory identification. The method was rigorously validated in accordance with accepted analytical guidelines. All 13 target alkaloids displayed excellent linearity within a mass concentration range of 0.2 to 50 ng/mL， with correlation coefficients （R²） consistently exceeding 0.99. The limits of detection （LODs） and limits of quantification （LOQs）， determined with acceptable accuracy and precision， ranged from 1 µg/kg to 4 µg/kg and 2 µg/kg to 10 µg/kg， respectively. Method accuracy and precision were assessed through recovery tests at three spiking levels， with six replicates at each level. The mean recoveries for all analytes ranged from 83.9% to 119.1%， with associated relative standard deviations （RSDs） all being ≤7.3%， validating the method’s high reliability and repeatability. Systematic evaluation indicated that the matrix effects for the 13 analytes were negligible； hence， the solvent calibration curve was adopted for quantitative analysis. The practical applicability of the validated method was demonstrated through the analysis of 50 batches of commercially available essential oil-based cosmetics. This market survey encompassed 15 products specifically marketed for infants or children and 35 products intended for adult use. As a result， none of the 13 target prohibited alkaloids were detected in any of the tested samples above their respective LOQs. A particularly notable accomplishment of this work is the successful development of a sensitive and reliable quantification strategy for oleandrin， a potent cardiotoxic alkaloid for which standardized detection methods in complex cosmetic matrices such as essential oils have been conspicuously absent. In conclusion， this study successfully establishes a simple， rapid， sensitive， and robust QuEChERS-UPLC-MS/MS method. It is comprehensively validated and clearly well-suited for the routine screening， risk monitoring， and quality control of 13 prohibited alkaloids in a wide array of essential oil-based cosmetics. The method offers reliable technical support for regulatory bodies to enforce safety standards and for manufacturers to ensure the safety and compliance of their products， thereby effectively contributing to the protection of consumer health.