Carbon-based shape-stabilized phase change materials (SSPCMs) are promising for thermal energy storage, but their performance is often limited by low PCM loading. Enhancing the compatibility between the PCM and supporting matrix is an effective approach to address this issue. Herein, biomass waste was converted into activated biochar supports through chemical treatment with different agents, a process that simultaneously regulated the construction of micro- and mesopores and enriched surface functional groups. This led to optimal compatibility and interaction with lauric acid (LA) and myristic acid (MA). The prepared SSPCM demonstrates excellent leak-free performance with a high PCM loading of 81%. The composite also exhibits a high thermal energy storage capacity, with melting and crystallization enthalpies of 134.0 J g⁻¹ and 106.4 J g⁻¹, respectively. Furthermore, an SSPCM-based coating displays effective temperature regulation in a simulated indoor environment. This work provides a new insight into the design of cost-effective, high-performance thermal storage materials.