To improve the energy of a kind of single base propellant (nitrocellulose/2, 4-dinitrotoluen/dibutyl phthalate/diphenylamine, NC/DNT/DBP/DPA), cyclotrimethylenetrinitramine (RDX) with different contents (5%, 10%, 15% and 20%) and different particle sizes (0.2 μm, 3.7 μm, 7.6 μm and 100.0 μm) were added into this single base propellant, and then the modified single base propellants containing RDX were prepared and obtained. The effect of content and particle size of RDX on the combustion performances of these modified single base propellants was studied by closed bomb experiments. The results show that: when the RDX particle size is 7.6 μm, with increasing the content of RDX, the burning rate of the modified single base propellant first decreases and then increases, and has a minimum value when the content of RDX is in the vicinity of 10%; the average values of the burning rate pressure exponent of modified single base propellants are all greater than 1 in the pressure range of 50-p_dpm MPa (p_dpm is the corresponding pressure when dp/dt reaches its maximum value); when the content of RDX is 5%, the burning rate of modified single base propellant decreases with decreasing the particle size of RDX, and in the pressure range of 50-p_dpm MPa, the average values of the burning rate pressure exponents of modified single base propellants with 0.2 μm and 3.7 μm RDX are both less than 1, while the mean values of the burning rate pressure exponent of modified single base propellants with 7.6 μm and 100.0 μm RDX are both greater than 1.