Metal hydride (MH) is an attractive alternative for thermochemical heat storage. This study proposes an optimal design methodology for MH heat storage reactors (MHHSRs), integrating the optimal design principle (ODP) and a new design procedure. Based on the optimal design methodology, the design of the powder bed with helical heat transfer fluid (HTF) tube is conducted via numerical simulation. A mathematical model is established for the thermal coupling between the powder bed and the HTF tube. The gravimetric exergy-output rate (GEOR) is adopted to evaluate the overall discharging performance. First, the helical HTF tube is optimized and improved based on the ODP, which increase the GEOR from 198.4 to 255.4 W kg(-1). The optimal helical diameter is 30 mm and the optimal improved structure is determined as a U-shaped double helical tube. Then, the structural improvement of the reaction bed is supplemented, achieving reductions of material and energy consumption by 12.2% and 11%, respectively. The final design of the powder bed with helical tube based on optimal design methodology improves the GEOR from 198.4 to 306.1 W kg(-1), which constitutes a significant increase of 54.3%. This optimal design methodology is validated and efficiently guides the design of advanced MHHSRs.