GAP-PDMH was prepared by the chemical reaction between alkyne and azide in DMF,and the molar ratio was 6/7. The reaction temperature was 60 ℃ and reaction time was 48 h. The GAP-PDMH was identified by FT-IR and ¹H NMR, and glass transition temperature of the GAP-PDMH was measured. Results show that PDMH is grafted into the branch of GAP chains by 1, 3-dipolar cycloaddition reaction between alkyne and azide. Mechanical properties of composite GAP propellant with GAP-PDMH were tested. Compared with the blank sample, GAP-PDMH can improve the mechanical properties of the composite propellant at room temperature. The maximum tensile strength is increased by 210% (to be 0.65 MPa); the maximum elongation is increased by 226% (to be 22.24%); the fracture elongation is increased by 207% (to be 25.87%).