To identify the foulants for ultrafiltration of Dongjiang River water, as well as their fouling process, hydrophobicity and molecular weight distribution of feed, permeate and chemical washing solution were analyzed both in rainy and dry season. High performance size exclusion chromatography (HPSEC) combined with peak fitting was employed as a tool to separate water sample into portions with different molecular weights. The results showed that biopolymers (98 ku), humic substances (1 200 u), building blocks (610 u) and low molecular-weight neutrals (270 u) were the main components in raw water from Dongjiang River, in which the amount of biopolymers and humic substances was higher in rainy season than that in dry season. More importantly, biopolymer and low molecular-weight neutrals were identified the main foulants in total and irreversible fouling during ultrafiltration of Dongjiang River water in both dry and rainy seasons. Furthermore, fitness between fouling data and four classic filtration modeling indicated that cake layer blocking by biopolymer and standard blocking by low molecular-weight neutrals caused fouling during ultrafiltration of Dongjiang River.