To improve the combustion and mechanical properties of ammonium perchlorate(AP)/composite modified double base(CMDB)propellant prepared by the solvent-extrusion method, the effects of carbon nanotubes(CNTs)on the thermal decomposition performances of the main components of AP/CMDB propellant and the microstructure of AP/CMDB propellant were studied by differential scanning calorimetry (DSC) and scanning electronic microscope(SEM). The burning rate and impact strength of AP/CMDB propellants with CNTs of mass fraction as 0.1%, 0.3% and 0.5% were tested. Results show that CNTs can effectively catalyze the thermal decomposition of AP, and make the low temperature decomposition peak of AP disappear, and the peak temperature of high temperature decomposition of AP shift 39.9 ℃ downwards, but it has little effect on the thermal decomposition of double base components NC and NG. CNTs can improve the microstructure of AP/CMDB propellant. There are no obvious micro cracks in the AP/CMDB propellant with the addition of 0.5% CNTs. CNTs can effectively improve the combustion and mechanical properties of AP / CMDB propellant. With increasing the content of CNTs the burning rate increases, the pressure exponent decreases, and the impact strength increases of the propellant. For AP/CMDB propellant with 0.5% CNTs, the burning rate at 10 MPa is 61.19 mm·s^-1, the pressure exponent between 10 MPa and 22 MPa is 0.51 and the impact strength is 5.55 kJ·m^-2 at -40 ℃.