The effect of diaphragm in grain on combustion performence of hybrid rocket engine was studied.The standard φ 100mm hybrid rocket engine of Beijing University of Aeronautics and Astronautics was selected as a computational model

and 98% hydrogen peroxide(H2O2)/hydroxyl terminated polybutadiene (HTPB) was combined as propellant;different hole diameters of diaphragm setting at different grain positions were computed by 2-D axial symmetry and gas-liquid two-phase flow model.The results show that smaller diameter of diaphragm hole and the diaphragm setting at positions ranging from 50% to 70% of grain length can obtain the largest averaged regression rate of fuel and the highest combustion efficiency.This conclusion will provide a theory evidence for design of the diaphragm.