In order to study the distribution characteristics and causes of heavy metals(HMs) in river sediment, six kinds of HMs(Cu, Zn, Cd, Cr, Pb and Ni) in sediment of the Xiaohe River were measured with atomic absorption spectrophotometer(AAS). The characteristics and causes of HMs pollution were analyzed and the potential ecological risk index method was used to assess the potential ecological risk coefficient and harm index of HMs in the river sediment. Results indicated that the content of HMs in sediment of the Xiaohe River was higher in the upper and middle basin, and lower in the lower basin under normal accumulation conditions. Under the non-normal accumulation conditions, such as channel fluctuation and new pollution sources, the content of HMs was higher in the lower basin than in the upper basin. The content of HMs in the river sediment was easily affected by water quality, and their relationship was analyzed by principal component analysis(PCA). Besides, the content of HMs in the river sediment was also affected by sediment size composition. Result from composite pollution evaluation for heavy metals in surface sediment of the Xiaohe River showed the order of Cd> Pb> Ni> Cu> Cr> Zn.