The dynamical stability, crystal structure, and mechanical properties of semi-titanium boride (Ti2B) are extensively studied using the pseudopotential plane-wave method within first-principles calculations. Soft transverse acoustic phonon modes are identified in the material, which has been assumed for a long time to have an Al2Cu-type structure with a I4/mcm space group. Using the frozen-phonon method, a tetragonal I4/m structure is uncovered to be energetically more stable and have similar diffraction patterns compared to the previously assumed Al2Cu-type structure. Phonon dispersion and elastic constant calculations reveal the dynamical and mechanical stability of the I4/m structure of Ti2B at ambient pressure. The calculated high elastic constants, modulus, and ideal strength show that Ti2B possesses outstanding mechanical properties.