For the system of water samples collected from Yangtze River,the effects of seasonal variation and Fe(Ⅲ) concentrations on the formation and distribution of trihalomethanes (THMs) during chlorination have been investigated.The corresponding lifetime cancer risk of the formed THMs to human beings was estimated using the parameters and procedure issued by the US EPA.The results indicated that the average concentration of THMs (100.81 μg/L) in spring was significantly higher than that in other seasons,which was related to the higher bromide ion concentration resulted from the intrusion of tidal saltwater.The total cancer risk in spring reached 8.23 × 10-5 and 8.86 × 10-5 for males and females,respectively,which were about two times of those in summer under the experimental conditions.Furthermore,it was found that the presence of Fe(Ⅲ) resulted in the increased level of THMs and greater cancer risk from exposure to humans.Under weak basic conditions,about 10％ of the increment of THMs from the water samples in spring was found in the presence of 0.5 mg/L Fe(Ⅲ) compared with the situation without Fe(Ⅲ).More attention should be given to the effect of the coexistence of Fe(Ⅲ) and bromide ions on the risk assessment of human intake of THMs from drinking water should be paid more attention,especially in the coastland and estuaries.