Polyethersulphone (PES)/sulfonated polysulphone (SPSf) blend ultrafiltration membrane was prepared via non-solvent induced gelation phase separation (NIGPS) using H2O as the non-solvent additive in the casting solution with N, N-dimethylacetamide (DMAc) as the solvent. An ultrasonic technique was employed to monitor membrane formation so as to provide the quantitative information during the phase separation. The effect of H2O concentration in the PES/SPSf/DMAc casting system on the membrane structure and properties was investigated. Results indicated that the viscosity of the casting solution increased with the increase of H2O content from 3 wt% to 9 wt% owing to the strong interaction between H2O molecules and the -SO3- group of SPSf chains, finally leading to the gelation of the system. Simultaneously, the microporous structure of the resultant membranes changed gradually into sponge-like structure from macrovoids with the increase of H2O content in the casting solution. It was related to the changes of the phase separation behavior from instantaneous demixing to delay demixing. Specifically, the approaching ratio could be used to predict the behaviors of the instantaneous and delay demixings. Moreover, the pure water flux of PES/SPSf blend membrane obtained under the conditions of polymer concentration 18 wt%, PES: SPSf = 84: 16 wt/wt and 9 wt% H2O in the casting solution was up to 858 L/m(2)h. The bovine serum albumin (BSA) rejection was 90.5%.