Smart polymers as ideal gene carriers have drawn increasing attentions due to the effective DNA release once triggered by intrinsic stimuli, as well as reduced cytotoxicity. Herein, a stimulus-responsive, positively charged and water-soluble polymer (OEI-TKx) was facilely engineered by cross-linking low molecular weight oligoethylenimine (OEI) via thioketal (TK) linkages that would cleave selectively in reactive oxygen species (ROS)-rich environments induced by hypoxia. Agarose gel electrophoresis assay demonstrated that the threshold N/P ratios for complete retardation of negatively charged DNA migration were above 5 for OEI-TKx. The reduction in DNA-condensing capability and the changes in particle size, size distribution and particle morphology all illustrated that OEI-TKx possessed excellent ROS responsiveness. OH-TKx/DNA polyplexes showed lower toxicity and higher gene transfection efficiency compared with PEI/DNA polyplexes. The optimum formulation, OEI-TKx/DNA polyplexes (N/P = 40), showed a little better performance than PEI/DNA polyplexes in cellular uptake profile. Furthermore, OEI-TKx/DNA polyplexes could escape from endosomes to the cytosol as efficiently as PEI/DNA polyplexes. Confocal images confirmed that OEI-TKx/DNA polyplexes could more effectively release DNA than PEI/DNA polyplexes, mainly owing to the valid cleavage of thioketal linkages induced by characteristic rich-ROS in Hela cells. These results suggested that OEI-TKx could represent an on-demand stimulus-responsive gene delivery platform.