The effects of ammonium perchlorate (AP),hexahydro-1,3,5-trinitro-s-triazine (RDX),aluminum powder (Al) and ammonium dinitramide (ADN) used as solid fillers on energy characteristics of composite PBT (BAMO and tetrahydrofuran (BAMO-THF) copolymers) propellants plasticized with A3 (bis(2,2-dinitropropyl)acetal/bis(2,2-dinitropropyl)formal) and NG/DEGDN (glyceryl trinitrate/diethyleneglycol dinitrate) respectively were investigated using least free energy method. Calculated results indicate that the variation curves of the specific impulse of propellants are characteristics of a parabola with the increasing of RDX content,and there exist optimum content ratios of solid fillers in respective systems. Compared with those plasticized with A3,propellants plasticized with NG/DEGDN show higher specific impulses. The specific impulses are evidently enhanced due to the decreasing in average relative molecular mass of combustion products,caused by the addition of 15% ADN instead of AP into the formulations.