A series of polyacrylate-polyurethane networks (PUA) were synthesized and their pH-sensitive behaviors were studied. An unexpected swelling-deswelling behavior was observed as the composition changed. Dynamic mechanical thermal analysis and differential scanning calorimetry were used to initially explain this phenomenon. By applying small-angle X-ray scattering and wide-angle X-ray diffraction simultaneously, a specific multiphase-transition process could be clearly detected, which made the mechanism of the observed abnormal pH sensitivity more explicit.