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A novel combined pretreatment strategy, where the hydrophobicity of feedwater is reduced and the morphology of flocs is optimized by varying the sequence of powdered activated carbon (PAC) adsorption and coagulation, is applied to alleviate membrane fouling. To explore the effects of pretreatment sequence and the role of PAC on the membrane fouling, the combined pretreatments of adsorption followed by coagulation (A-C) and coagulation followed by adsorption (C-A) are investigated with various PAC dosages (10, 20, and 50 mg/L). The results reveal that since PAC directly participates in the growth of flocs as the core during A-C pretreatment, the A-C pretreatment is more conducive than the C-A pretreatment to form flocs with a larger size, higher strength, and smaller fractal dimension. Thus, the flocs can accumulate into a porous cake layer to improve the membrane permeability and fouling reversibility. Moreover, the irreversible fouling is highly linearly correlated SUVA values in the early ultrafiltration cycles (R2 SUVA = 0.9536) and with the floc strength in the late cycles (R2 Strength = 0.9348–0.9763). This finding can be used to make a targeted diagnosis of irreversible fouling in different cycles, providing valuable guidance for prolonging the service life of ultrafiltration membranes. [Display omitted] • The sequence of combined pretreatment has significant effects on membrane fouling. • PAC has a higher adsorption efficiency for organic matter in A-C pretreatment. • Flocs with PAC as the core have a larger size, higher strength, and lower D f. • Irreversible fouling (IF) can be selectively predicted by SUVA and floc strength. • A-C pretreatment promotes the formation of a porous cake layer to alleviate IF. [ABSTRACT FROM AUTHOR]