Polycationic gene carriers, mPEG-b-PLL-g-succinyl sulfathiazole (denoted as PPS), were synthesized and characterized as a shielding system for cationic gene delivery. DNA/PEI/PPS ternary complexes were prepared by shielding PPS to DNA/PEI polyplex solution. PPS can shield the surface charge of DNA/PEI polyplexes from positive (27.7 mV) to negative (-24.6 mV), and the acidic condition should trigger the disassembly of ternary complex. The gene transfection efficiency of DNA/PEI/PPS was lower than DNA/PEI at pH 7.4, but at pH 6.0, it recovered to the same level as DNA/PEI. CLSM results showed the same trend as transfection at different pH. The suitable pH-triggered shielding/deshielding ability estimated that PPS should be a potential shielding system to be used in in vivo gene delivery.