The work aims to improve the environmental friendliness and grease resistance of the packaging paper. The dialdehyde chitosan (D-CTS) was prepared from the selective oxidized chitosan through sodium periodate. Sizing agent D-CTS/CS was obtained by mixing dialdehyde chitosan (D-CTS) with cationic starch (CS). Then, the grease resistance of the coated paper was tested by applying the sizing agent D-CTS/CS on the handmade paper. The surface contact angle instrument was adopted to analyze the function mechanism of grease resistance. In the process of mixing the sizing agent, when the mass fraction of dialdehyde chitosan was 0.3%, drying temperature was 80 ℃, drying time was 50 s and pH was 5.5～6.5, the grease resistance of coated paper was the best and the maximum grease resistance index could reach 10. After the paper surface is processed with sizing agent, both the electrostatic interaction of paper surface and the film-forming property are improved. In such case, the oil drop can be prevented from permeating into the paper, so that the grease resistance will be strengthened.