In order to clarify the effect of multi-modal ammonium perchlorate（AP ）on combustion responses characteristics of AP/Hydroxyl polybutadiene(HTPB) composite propellants, the unsteady combustion response (Rp) model of multi-modal AP composite propellant has been established and calibrated. Under 10 MPa and 25-1000 Hz conditions, more than 10 types of AP-based propellant samples containing AP with four typical average particle sizes (330， 250，110 μm and 50 μm) have been used to calculate the characteristics of the combustion response. The effects of the AP modal and mass ratio on combustion response function have been analyzed. The results show that: 1) the Rp model calculation results are in good agreement with experimental results reported in the literature (pressure >5 MPa), deviation <9%; 2) the AP particle modal and mass ratio have great effects on the combustion response function, and law is basically satisfied, showing that the increased content of small-sized AP or reduced the content of large-size AP could reduce medium and low frequency oscillation, but meanwhile it would drive higher frequency oscillation; 3) Replacement of small-sized AP with medium-sized AP can reduce medium and high frequency oscillation. Conversely, the replacement of the large-size AP with medium-size AP can reduced low frequency oscillation.