Based on three kinds of fly ash content (15%, 20%, 25%) and three kinds of cement content (6%, 8%, 10%), cubic specimens with side lengths of 40, 50, 70.7 and 100 mm were prepared to investigate the effect of size on the damage characteristics and energy evolution of the filled bodies. The results show that when the size of the filling body is certain, the strength of the filling body with 20% fly ash content and 10% cement content is the highest, and the compressive strength of the filling body shows the law of “increasing first and then decreasing” with the increase of the size, and it reaches the peak value of 19.85MPa when the size is 70.7mm. The modulus of elasticity of the filling body was positively correlated with the size, and with the increase of the specimen size, the damage form changed from tensile damage to X-type shear damage. The energy characteristics of filling bodies of different sizes under uniaxial compression are similar, the elastic energy dominates before reaching the maximum stress, and the dissipated energy rises after reaching the maximum stress, and with the increase of size, the energy storage limit and bearing capacity of the filling body show the trend of increasing and then decreasing, and the curve of elastic-energy consumption ratio changes from stable to rising, which can be used as a basis for the destruction of the filling body.