Owing to their unique electronic structures and metallic-like properties, transition metal borides (TMBs) have demonstrated activity and stability that surpass those of traditional catalysts in the hydrogen evolution reaction (HER) of water splitting, becoming a research focus in the energy materials field. However, existing research generally lacks a systematic decoupling of the multidimensional correlation mechanisms of synthetic methods, structural regulation, and performance optimisation, severely restricting the rational design process of TMB catalysts. The aim of this review is to provide a cross-scale design paradigm for the development of high-performance TMB-based HER electrocatalysts by constructing a three-in-one analytical framework of theoretical guidance, synthetic innovation, and mechanism analysis. First, based on a fundamental understanding of the HER mechanism and d-band theory, we propose core principles for designing efficient catalysts. We review various synthetic methods, from traditional methods to innovative methods, and discuss their impact on catalytic performance. Through an in-depth analysis of the correlation between synthetic parameters and HER activity, valuable insights are provided for researchers seeking to optimise TMB-based electrocatalysts. Finally, this review highlights the current challenges and outlines future directions, emphasising the immense potential of TMB-based electrocatalysts in advancing sustainable hydrogen production.