In order to investigate the gel breaking and degradation behavior of cationic polyacrylamide thickener in acid solutions at higher temperature，poly（methacryloyloxyethyl trimethyl ammonium chloride）PDMC， poly（acrylamide）PAM and P（DMC-AM） with ionic degree of 100.0%， 0.0%， 49.1%，45.9% and 12.7% ，respectively，were synthesized under the condition of pH 7.5 and the temperature of 40~60℃ with the DMC/AM monomer mass ratio of 12/0，0/12，9/3，6/6，3/9，respectively. The change of solution viscosity of P（DMC-AM）with different cationic degree before and after degradation was explored，and the structural change of P（DMC-AM）before and after degradation were analyzed by IR and 1H-NMR. The results showed that when the five samples were dissolved in acidic liquid（2% or 10% hydrochloric acid）with the mass fraction of 1%，and then was leaved at the temperature of 120℃ for 24 h or 48 h，with the increase of ionic degree，the viscosity of P（DMC-AM）solution increased slightly after degradation in general. The steric hindrance effect of cationic side chain could inhibit the degradation，and the higher ionic degree could inhibit gel-breaking. During the degradation process，P（DMC-AM）chiefly occurred backbone scission accompanied by ester bond and amide bond cleavage. Moreover，the degradation of copolymer increased with the increase of the concentration of hydrochloric acid.