Various phases of TiH_x (x=1-2) were constructed, with all possible H-atom configurations being considered. The first-principles energetics calculations were performed to evaluate their formation preference and mechanical and thermodynamic stabilities. All the results were then combined to construct the stable and metastable phase stability diagrams of TiH_x. It is suggested that many stable and metastable TiH_x phases have very comparable formation energies and thus are likely to co-exist. Their relative stabilities are strongly depended on temperature (T) and the partial pressure of hydrogen (p(H₂)). Over the entire T and p(H₂) range of interest, γ-TiH and γ-TiH₂ are the only two stable phases. ε-TiH, γ-TiH_1.5, ε- and δ-TiH_1.75, and ε-TiH₂ can possibly present as metastable phases. These metastable phases are all mechanically stable, and have a thermodynamic tendency to continuously absorb or release H until reaching the equilibrium phases of γ-TiH₂ or γ-TiH.