Abstract: The Mg-Ti-Mn alloys exhibit excellent hydrogen desorption properties owing to the presence of the catalytic phases α-Ti, α-Mn and TiMn2. However, the phase equilibria in the Mg-rich corner remain uncertain. To clarify the phase equilibria in the Mg-rich corner of the Mg-Ti-Mn-H system, thermodynamic databases of the Ti-Mn, Ti-H and Ti-Mn-H systems were optimized based on the experimental data obtained in the present work and from the literature. The isothermal section of Mg-Ti-Mn ternary system at 773 K was verified using three key alloys. By extrapolating the Mg-Ti-Mn-H thermodynamic model, the calculated Pressure-Composition-Temperature (P-C-T) curves showed a deviation of only 2.1% from the experimental data, confirming the reliability of the established database. Utilizing the developed Mg-Ti-Mn-H database, four Mg-Ti-Mn alloys were designed to evaluate the hydrogen absorption/desorption kinetics. The Mg84.5Ti2.8Mn12.7 (consisting of α-Mg + α-Mn + TiMn2) exhibited the best hydrogen storage properties, which absorbed 4.53 wt.% hydrogen in 90 min and desorbed 5.16 wt.% hydrogen in 20 min at 573 K. This work provides guidance for the composition design and phase selection of the Mg-Ti-Mn hydrogen storage alloys.