The critical flux of submerged flat membrane bioreactor (MBR) was measured by an improved fluxstepwise method and the measured values under different relaxation time were analyzed.The result was that with longer relaxation time,asymmetry of FluxTMP and FluxPermeability decreases remarkably with much lower membrane residual resistance and better fouling reversibility in subcritical flux zone and without obvious hysteresis.The relaxationzoneattached fluxstepwise method can better describe the principle of fluxpermeability and alleviate the impact of reversible fouling.