Abstract: This paper was designed to study the tendency of membrane fouling by using membrane critical flux． The response surface model was used to study different factors，such as sludge concentration，space between membranes and aeration rate， affecting membrane fouling of the upper and lower layer membrane module． It was found that the model was fitting and significant，moreover，the sludge concentration，space between membranes and aeration rate had a significant impact on the upper and lower membrane fouling． Meanwhile，it was also observed that the critical flux of both upper and lower layer membrane module sharply decreased with the increase of sludge concentration． However，the different variation tendency of membrane fouling between upper and lower layer membrane module was detected due to the change of space between membranes and aeration rate under different sludge concentration． Finally，optimum operating parameters under different sludge concentration simulated by response surface model were successfully applied to the process of using flat-sheet membrane for four-stage sludge thickening．