Infrared spectraphotometry was used for determining petroleum substance in groundwater. Petroleum substance in groundwater was extracted by carbon tetrachloride, adsorbed by magnesium silicate, then detected after removing polar substances. The detection limit was 0.014 mg/L,and the relative standard deviation of the detection results was in the range of 0.65%–1.18%(n=l1). The recoveries of standard additions were 84.0%and 93.2%. The method is sensitive,simple,rapid,and it can meet the analysis demand of the petroleum class in groundwater.