Abstract: In recent years, non-viral gene vectors have attracted great attention for efficient gene delivery due to the advantages, including low toxicity, low immunogenicity and simple preparation. Polyethylenimine (PEI) is one of the typical non-viral gene carriers that have been widely utilized for gene delivery owing to its superior capabilities in gene compression and buffering capacity. This article discusses the processes of gene delivery and the barriers of PEI-based carrier during the gene delivery, such as low biocompatibility, cytotoxicity, lack of specific targeting and insufficient gene release, etc. Therefore, we summarize the multiple approaches for the modifications of PEI in terms of improved biocompatibility, degradability, specific targeting and buffering capacity. Furthermore, we also review on the recent impressive progresses of smart stimuli-responsive PEI carriers, including endogenous stimuli (pH, reactive oxygen species, glutathione, biomolecular, etc), exogenous stimuli (light, temperature, magnetic field, etc) and dual-responsive strategies, which might provide guidance for the development of more efficient and safer non-viral gene vectors.