Polyacrylic acid sodium (PAAS) was applied to remove Zn (II) from aqueous solutions by complexationultrafiltration using a rotating disk membrane module. As important factors, solution pH and the mass ratio of polymer to metal ions (P/M) on the rejection of Zn (II) were investigated, and the rejection could reach to 95.3% at pH=7, P/M=25 and 500 rpm. In addition, a partition model was proposed to reveal instability mechanism of the PAA-Zn complex in the shear field for the first time. The critical rotating speeds at which PAA-Zn complexes begin to dissociate were 1,460, 1,390, 1,280 rpm at pH 7.0, 6.0, 5.0, respectively. The corresponding critical shear rates (γc), the smallest shear rates at which PAA-Zn complex begins to dissociate, were 1.58 X 10 5, 1.45 X 10 5 and 1.25 X 10 5 s-1 at pH 7.0, 6.0, 5.0, respectively. In addition, the relationship between the critical radii and the critical rotating speed was obtained.