This article introduces the current research progress and theoretical basis of designing the TWIP / TRIP (TRIP: transformation induced plasticity; TWIP: twin induced plasticity) metastable β titanium alloys, and summarizes β-grain sizes, yield strength and work hardening capability achieved in the double twined {332} <113> and {112} <111> metastable β titanium alloys. The application and limitation of Bo-Md diagram in the design of multi-component titanium alloys are further discussed, especially the effect of different secondary phases (α-phase and ω-phase) on the stability of the matrix β-phase. The factors that influence the deformation mechanisms of TWIP / TRIP metastable β titanium alloys are emphasized in terms of the matrix β phase stability, precipitated phases, β-grain sizes and crystallographic orientations, in which the current deficiencies of this field are analyzed. Furthermore, the effect of double twinning mechanisms on the mechanical properties of titanium alloys is briefly discussed as well. Finally, by summarizing the research progress and related issues, this article conveys the new insights into the development of high-strength and tough titanium alloys.