To ensure safe and stable secondary frequency regulation operation in thermal power units assisted by hybrid energy storage systems while boosting economic efficiency， this paper proposes a wavelet transform-based method for secondary frequency regulation optimal capacity allocation. First， a hybrid energy storage secondary frequency regulation system model integrating thermal power units， battery energy storage， and compressed air energy storage is established. Second， the wavelet transform-based approach for optimally configuring secondary frequency regulation capacity is proposed， encompassing both power allocation and capacity configuration for battery and compressed air energy storage. Finally， a comprehensive cost-benefit model and evaluation indices are introduced to analyze the economic benefits and frequency regulation performance of the hybrid energy storage optimal configuration. A simulation model for hybrid energy storage secondary frequency regulation is built in simulation software to validate the method’s effectiveness.