By means of the statistical data of valence electron structure parameters of metallic phase and phase interfaces combined with the preparation technology and phase transformation, the theoretical calculation formulae for the tensile strength of high strength titanium alloys were put forward, i.e., the tensile strength of high strength titanium alloys is the sum of strength of b-Ti and the strengthening strength increments caused by solid-solution strengthening and interface strengthening including precipitation strengthening of alloying element. Based on the experimental data of the high strength metastable b titanium alloy Ti-B20 developed by Northwest Institute for Nonferrous Metal Research, its tensile strength was calculated when it was treated by solid solution above b transus, below b transus and at 780 °C followed by aging, respectively. The calculation results agree well with the experimental results.