In order to study the sources of groundwater chemical components and the environmental health risks along the Le’an River in Leping City under the effect of an extreme flood event, 22 parameters of 24 groups of shallow groundwater samples collected during and after an extreme flood were analyzed by the combination of graphical method, mathematical statistics and main ion ratio. The results show that there is no significant difference in the main chemical components of groundwater in different periods, and all the samples are tested to be weak acidic fresh water with low salinity, with HCO-3, SO42-, Ca2+, Na+ and K+ as the dominant anions and cations. The spatial distribution of groundwater chemical types is significantly differntiated, which is mainly HCO3-Ca·Na and HCO3·SO4-Ca·Na types during the flood period, and Cl and NO3 types in the post flood period. The formation of the main chemical components of groundwater is mainly controlled by the water-rock interaction, the dissolution of silicate minerals and the cation alternating adsorption. In additon, some local areas are also affected by human activites such as agricultural fertilization, sewage discharge. After the flood, the groundwater quality is degraded, which is no loger suitable for drinking. The main pollutants are ammonia nitrogen, NO3-, Mn and Cd, which come from the surface-groundwater mixing, the leaching of manganese-containing minerals in the soil by the flood and the mining activities in the upper reaches of Le’an River.