The strategy of under-dosing coagulant was firstly applied in a coagulation-ultrafiltration (UF) process for treatment of humic-rich water in this study. When polyaluminum chloride (PACl) dosage was set as 0.025 mM (as alum), the removal efficiency of organic matter and residual turbidity were slightly inferior to 0.1 mM (as alum), but four times dosage lowered than the optimal-dosing. Although permeate flux of ultrafiltration membrane was larger at the optimal-doing than that of under-dosing, the less proportion of irreversible fouling and large proportion of reversible fouling were proved that under-dosing strategy has more meaningful engineering application value for deep investigation. The impacts of Ca2+ on coagulation and ultrafiltration performance are also investigated under different coagulant doses. Removal of humic-like substance, floc size and incompactness were appreciably enhanced at under-dose with increasing Ca2+ concentration, but did not distinctly change at the optimal-dosing. Besides, lower positive charge of flocs produced at under-dosing hindered the foulants transport to membrane surface. The interactions between humic substance, positively-charged coagulants, and structure of cake layer firstly systemically investigated the impacts on the performance of ultrafiltration in our study. The results shown that the technology of under-dosing with addition of Ca2+ has dramatically increased the UV254 removal efficiency, formed loose and porous flocs still outside of the critical electrostatic distance of membrane surface, which significantly remit the membrane fouling and improve the reversibility of membrane fouling. (C) 2015 Elsevier B.V. All rights reserved.