In this paper, flocculating performance and mechanisms of a new composite coagulant, poly-ferric aluminum chloride–polydimethyl diallylammonium chloride (PFAC-PD) with different OH−/(Fe3+ + Al3+) molar ratios, were investigated for humic acid (HA)–kaolin synthetic wastewater treatment. The impact of OH−/(Fe3+ + Al3+) molar ratios on the removal efficiencies of turbidity and dissolved organic carbon, specific UV absorbance, coagulation mechanisms and dynamics was explored during the coagulation process using composite coagulants. The coagulation experimental results revealed that the composite coagulants with lower OH−/(Fe3+ + Al3+) molar ratio exhibited better coagulation efficiency. When OH−/(Fe3+ + Al3+) molar ratio of the composite coagulant was 1.5, adsorption-bridging played a dominant role in coagulating HA–kaolin synthetic wastewater. The floc growth rate and floc size, increased with increasing OH−/(Fe3+ + Al3+) molar ratio and the highest peak height of the size distribution was obtained by PFAC-PD with OH−/(Fe3+ + Al3+) = 1.5. Also, the composite coagulants with higher OH−/(Fe3+ + Al3+) molar ratio formed more compact flocs, as reflected by the higher fractal dimension value. The flocs coagulated by PFAC-PD with basicity value of 1.0 gave strong strength and good recoverability.