Abstract：One of the important methods for studying the formation mechanism of groundwater with high arsenic concentration is to analyze the content and modes of occurrence of arsenic in sediments and to understand how arsenic is dissolved in groundwater from the viewpoint of water-rock interaction. In this paper, the authors mainly concentrated on the study of the modes of occurrence of arsenic in Quaternary sediments of the Hetao Plain, based on the analyses of rock and soil samples and dissolution tests on different media including water, acid and alkali. The results show that arsenic mainly exists in the form of non-crystalline colloid in Quaternary sediments of the Hetao Plain. In the sediments, a positive correlation between the content of arsenic and that of iron in sediments is obviously observed. The analytical results of EDS equipped with X-ray meter and electronic microscope reveal that no arsenic mineral was found in the samples taken from different depths and different lithologic characters in the arsenic contamination area. There is no apparent correlation between the content of organic matter and that of arsenic in sediments. In addition, the content of organic matter is very low. Arsenic in sediments is relatively easily dissolved in alkaline solution. The dissolved ratios are 87.5%-98.75% for arsenic in sandy sediments and 72.73%-84.21% for arsenic in clay sediments. However, for arsenic in pyrite mineral (with arsenic content being 87.30 mg/kg), the dissolved ratio is only 0.02%.