Energy density, which is an important indicator of the performance of solid propellants, is known to increase with the addition of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20). However, it remains unclear how CL-20 affects the decomposition of ammonium perchlorate (AP) and energy release. Here, the effects of CL-20 on the combustion performance and agglomeration of propellants were investigated. The addition of CL-20 decreased AP decomposition temperature and the energy required for the transformation of AP crystals from orthorhombic to cubic. The burning rate and pressure exponent of the propellant with 42% CL-20 were significantly higher than those of the propellant containing 20% CL-20. Thus, adding CL-20 to the propellant improves the energy characteristics and burning rate and the pressure exponent increases. At low combustion chamber pressure, the agglomeration of the propellant containing a high content of CL-20 will be blown away from the combustion surface only after staying on that surface for a short time. In this process, the probability of volume growth of the agglomeration after merging with other agglomerations greatly decreases, thus reducing the overall agglomerate particle sizes; further, the addition of a small amount of CL-20 to the propellant may lead to a reduction in agglomerate particle sizes. AP with a smaller particle size weakens the agglomeration in the combustion process and decreases the number of agglomerates with large particle sizes. These findings lay the foundation for the development of novel high-energy propellants.