The coagulation experiment, with Kaolin as objects, aluminum chloride (PAC) as coagulant and hydrated MnO2 as coagulant aid, were accomplished under different conditions. In the experiment, the particle size distribution and turbidity in water were detected by on-line detector. The results show that increase PAC dosage, original turbidity, hydrated MnO2 dosage and coagulation time will make the fractal dimensions of floc growth in micro-coagulation stage increase. The fractal dimensions of floc growth in micro-coagulation stage increasing means more particle size <5µm flocs are removed. Hydrated MnO2 can strengthen micro-coagulation.