To improve the re-growth ability of broken flocs and the removals of colloids and small particles, the raw water with character of low temperature and turbidity was investigated using optical detection instruments and mathematical method. The effects of additional dosage of PAC on the variations of the floc size, growth rate and floc size distribution were studied and the removals of turbidity and particles were also conducted. With the increase of PAC dosage, the floc size and growth rate showed an initial upward trend followed by a downward trend, and the optimum dosage for the floc size and growth rate was 4 mg／L and 6 mg／L, respectively. Moreover, the residual turbidity and particles were significantly decreased first, and then increased. The lowest values of residual turbidity and particles were 0.79 NTU and 90 mL-1. When the addition dosage was 2 mg／L, the volume percentage of the flocs in the range of 280-630 μm was evidently increased from 22.85 ％ to 46.36 ％, while that of the flocs in the range of 4-35 μm was decreased from 1.21％ to 0.19％. In addition, the floc size after re-growth was 400 μm, larger than that before breakage, with the growth rate of 77.26 μm／min. Therefore, the additional dosage of PAC could obviously improve the re-growth ability of the broken flocs and the removals of turbidity and particles. Moreover, the experimental results could effectively improve the treatment efficiency and water quality.