Al13 or Alb is usually regarded as the most efficient species of polyaluminum chloride (PAC), the performance flocculant for water treatment. This paper was intended to report a new method to synthesize PAC with high content Alb, by using the membrane reactor. NaOH solutions were managed to permeate slowly through the micropores of ultrafiltration membrane into AICl3 solutions under the suitable transmembrane pressure(TMP). Meanwhile NaOH drops size was limited to nano-scale, resulting in dramatical reduction of the characteristic diffusion time and great increment of contact interface between the strong base and Al ions in solution to favor the formation of Al(OH)4-, the precursor of Al13, so few precipitates and much Alb are produced. When the initial concentration of AlCl3/NaOH is 0.40/2.0 (mol/L), MWCO = 10000, TMP = 0.0085 MPa, T = 305 K and B (molar ratio of OH-/Al3+) = 2.25, the quantity of Alb attains about 80%. The results of 27Al-NMR determination showed that the Al13 content is equal to Alb content. And our PAC product has shown better flocculation effects than the commercial product.