In this paper, biodegradable polycations based on polycarbonates with grafted polyethylenimine (PEI) were synthesized as a nonviral vector or gene delivery. Immobilized porcine pancreas lipase (IPPL) was employed 19 perform the copolymerization of 5-methyl-5-allyloxy carbonyl-trimethylenecarbonate (MAC) with 5,5-dimethyl-trimethylene carbonate (DTC). The DTC molar percent X was equal to 6.7, 12.5, and 45.4, respectively. The resulting copolymers with different compositions (P(MAC-co-DTCx) underwent additional allyl epoxidation and thereby grafted by low molecular weight PEI1800. The MWs of P(MAC-co-DTCx)-g-PEI, measured by GPC-MALLS, were 219800, 179100, and 51700 g/mol with polydispersities of 1.5, 1.4, and 1.2, respectively. Physicochemical properties of these vectors were characterized and the DNA loading was evaluated. P(MAC-co-DTCx)-g-PEI could form nanosized particles (less than 100 nm) with pDNA. The three P(MAC-co-DTCx)-g-PEI/DNA polyplexes had similar buffer capabilities that were better than that of PE125K and PMAC-g-PEI. Despite a slightly lower DNA binding ability, the PEI-grafted polycarbonates, especially P(MAC-co-DTC45.4)-g-PEI, presented apparently low cytotoxicity and much higher gene transfection efficiency in comparison with PEI25K in 293T cells. Moreover, preincubation of P(MAC-co-DTC6.7)-g-PEI showed a quickly weakening DNA binding capacity, while a suitable degradation rate of vectors would facilitate the efficient release of pDNA from polyplexes after cellular uptake and also reduce cell cytotoxicity. The results of this study demonstrated the promise of P(MAC-co-DTCx)-g-PEI copolymers for efficient gene delivery.