The cycling stability of ZrCo hydrogen isotope storage alloys in fusion reactors is crucial for their engineering applications. In this work, we comprehensively studied the relationship between the hydrogen-induced disproportionation reaction and the phase transformation process during cycling to understand the capacity attenuation of the ZrCo alloy. It is demonstrated that a non-isomorphic transformation between the α (ZrCo) and β (ZrCoH3) phase leads to the attenuation of cycling capacity. Thus, we establish an isomorphic transformation between the α phase and the αH phase through pressure control (e.g., 5 bar H2), which exhibits an astonishing capacity retention rate of 99.5% even at a high temperature (415 °C). It is worth mentioning that our work achieved a stable hydrogen absorption and desorption cycle in the ZrCo alloy at high temperatures without any element doping, which maintains the original platform characteristics of the ZrCo alloy well and is of great significance for engineering applications.