A novel semi-interpenetrating polymer network (semi-IPN) membrane composed of crosslinked chitosan (cr-CS) and poly( acrylic acid) (PAA) was prepared. Evidence from infrared spectra proved the formation of polyelectrolyte complex through electrostatic interaction between NH3+ groups from CS and COO- groups from PAA. The semi-IPN membrane swelled at high pH and at low pH exhibited a typical pH-sensitivity. Its swelling degrees in different salt solutions with the same ionic valence and equal ionic strength were on similar levels. Under certain ionic strength (I = 1.5 mol/L), the degree of swelling increased with increased metal ionic valence. Furthermore, the elongation of the semi-IPN membrane could vary reversibly by immersion into CaCl2 solution and KCl solution alternately. Reasons for the chemomechanical behavior are discussed.