This study was designed to reverse multidrug resistance of lung cancer cells by downregulating MDR1 genes through RNA interference (RNAi) technology. A novel biodegradable cationic polymer (PEG- b- PLG-g-PEIs, GGI) was synthesized and characterized by 1H NMR. The particle size and zeta potential were measured by dynamic light scattering (DLS). The cell viability profile of GGI was tested by MTT method with both A549 and A549/DDP cell lines. Flow cytometry (FCM) technology was used to investigate the efficiency and intensity of delivering siRNA to cells by GGI polymer. RT-PCR and Western blot were used to detect the mRNA and P-gp expression after GGI/MDR1 siRNA transfection assay. The sensitivity of cisplatin administration after transfecting GGI/MDR1 siRNA polyplexs was performed with MTT and Annexin V-FITC/PI methods. The results suggest that the particle size and zeta potential of GGI/siRNA were 150 −200 nm and 16−28 mV. GGI exhibited a lower cell cytotoxity than PEI 25K and higher efficiency of delivering siRNA, which dramatically decreased the expression of MDR1 mRNA and P-gp of A549/DDP cells and increased much sensitivity to cisplatin in A549/DDP cells. GGI holds a great potential in gene delivery as a novel cationic polymer for further investigation.