Polyaluminum Silicate Chloride (PASiC) is a new water treatment coagulant. It contains silica that can be hydrolyzed to produce SiO2 as coagulant aid. Hence, this coagulant consumes little alkalinity and is effective at low dosages. It is, therefore, especially suitable for treating water of low alkalinity and low turbidity. When manufacturing PASiC, the alkalinity solution involved in the polymeric reaction must be added slowly to the aluminum salt solution to avoid local over-saturation which enhances the production of high-valence medium polymer species (Alb) (e.g. Al12AlO4(OH)(24)(7+), Al-13). In this research, the use of an ultrasonic vibrating system to assist the alkalinity dosing during the formation of PASiC was studied. The ultrasonic vibrating system breaks up the NaOH solution into fine mists so that the latter can be more evenly dispersed into the aluminum silica solution to avoid local over-saturation. This ultrasonic-assisted NaOH dosing method leads to a more stable polymerization of aluminum changing ratios of Ala, Alb, and Alc in the final product and raising the Alb portion in the coagulant. Long-term observations also show that PASiC prepared with the ultrasonic-aided NaOH dosing at 0.25 mL/min is more effective in removing turbidity.