Abstract: In this article, the microwave digestion method was used to pretreat the environmental water samples, and the inductively coupled plasma mass spectrometry(ICP-MS) was used to rapidly determine the thorium content in the environmental water samples. The measurement and microwave digestion conditions were optimized. The results were analyzed in the aspects of selection of internal standard, detection limits, precision, accuracy, recovery, and actual sample measurements. The experimental results show that the relative deviation of measurement when analyzing thorium in water with 209Bi as the internal standard is the smallest, 0.2%-1.3%; the detection limit of this method is 0.003 μg/L. The precision of the method was investigated. The relative standard deviation(s_r) of the determination results is less than 6.0%(n=6). The reference material measurement and spiked recovery experiments were carried out at three different concentration levels. The measured values are basically consistent with the certified values, and the recoveries are 93.4%-106.2%. Twenty actual environmental water samples were measured. The results are within the range of results measured in 2016, which verifies the practicability of this method in measuring thorium content in environmental water samples.