Tumors have emerged as the number one cause of death with the incidence rate rising rapidly due to various factors. Nanoparticle-mediated PolyI:C (PIC) delivery is a promising approach in the treatment of various tumors. However, the delivery of PIC to target cells is very difficult. Therefore, it is necessary to develop carriers which can safely deliver PIC to the target cells. In this study, chitosan-g-poly (ethylene glycol)-g-polyethyleneimine copolymer (CS-g-PEG-g-PEI) which delivers PIC as a carrier was synthesized by carbonyldiimidazole (CDI) coupling method using ionic liquid as the reaction medium. The results of Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy (1H NMR and 13C NMR), elemental analysis and gel permeation chromatography (GPC) showed that PEG and PEI were grafted onto CS successfully. The result of cytotoxicity assay indicated low levels of cytotoxicity in CS-g-PEG-g-PEI. The positively charged CS-g-PEG-g-PEI self-assembled with the negatively charged PIC to form composite nanoparticles whose physicochemical properties were studied. The results of stability experiments and subsequent gel electrophoresis indicated that the stability of nanoparticles solution was good with water as a solvent, CS-g-PEG-g-PEI as a carrier, the ratio of nitrogen/phosphorus (N/P) is 8, and the concentration of PIC as 0.5 mg/mL. CS-g-PEG-g-PEI could bind to PIC almost completely and effectively protect it from degradation of Rnase A at N/P=8, and the PIC encapsulation efficiency was over 87%. The results of morphological characterization showed that nano-sized self-assembled aggregates with core-shell structure could be prepared successfully. This provides a new carrier for long-circulation and research of solid tumor drug delivery system.