Abstract: Fully homogenized sample (1.0 g) was extracted with 4 mL of 80% (volume fraction, the same below) methanol solution, whirled for 2 min, sonicated for 10 min, and centrifuged freezingly for 5 min at 4 ℃, and the extraction was made once again. The supernatant was combined, and diluted to 10 mL with 80% methanol solution. An aliquot (5 mL) was taken and mixed with 20 mL of phosphate buffer solution (pH 7.3). After vortexing, the above solution was passed through the immunoaffinity column activated beforehand, and the column was rinsed with 3 mL of 20% (volume fraction) methanol solution, drained under pressure, and eluted with 3 mL of methanol. During the purification process, the temperature was controlled at room temperature, and the column passing rate was set to 2-3 mL·min^-1. The eluate was collected, and an aliquot (1.0-1.5 mL) was centrifuged for 10 min. The supernatant was determined by ultra-high performance liquid chromatography-triple quadrupole composite linear ion trap mass spectrometry. Poroshell 120 EC-C₁₈ column was used as the stationary phase, and gradient elution was performed with the mixture of 10 mmol·L^-1 ammonium formate-0.1% (volume fraction) formic acid solution and methanol at various volume ratios as mobile phase. Multiple reaction monitoring-information dependent acquisition-enhanced production ion (MRM-IDA-EPI) scanning mode was used for obtaining the quantitative results and completing the matching of secondary mass spectrum. It was shown that the linear relationship between values of the peak area and the mass concentration of brevetoxin was kept in the range of 5-500 μg·L^-1, with detection limit (3S/N) of 30 μg·kg^-1. Test for recovery was made on the negative sample by standard addition method, giving recoveries in the range of 80.5%-108%, and RSDs (n=6) of the determined values ranged from 3.1% to 6.0%. The proposed method was applied to the analysis of bivalves, and brevetoxin was not detected. Values of the matching degree, reverse matching degree and purity between EPI secondary mass spectrum of the target in the spiked sample and standard secondary mass spectrum were above 60%, and it was turned out that a high similarity was obtained between the target and brevetoxin.