The influence of composition on mechanical properties of fuel-rich composite propellants based on hydroxyl-terminated polybutadiene pre-polymer (HTPB)/ammonium perchlorate oxidizer (AP)/aluminum fuel (Al) was experimentally investigated. 11 Batches of 10 kg mixes were prepared in which design factors of mixing, casting, and curing were fixed. Standard JANNAF specimens were tested with Zwick machine at a rate of 50 mm·min^-1 under 25 ℃. The results show that the decreasing of the binder content from 74.5% to 34.5% due to fillers incorporation such as AP an Al powder decreased the strain and increased the tensile strength, Young′s modulus, and hardness. The particle size and amount of AP had influences on the mechanical properties. The fine AP particles acted as active filler and enhanced the mechanical properties of composite propellant. Decreasing AP average particle diameter from 55.9μm to 9μm was helpful to increase the tensile strength, Young′s modulus, and hardness of the propellant and to decrease the strain. It is indicated that the mechanical properties of these fuel-rich composite propellant compositions are mainly affected by the content of the binder and the average particle size of AP.