To study the relationship between the microstructure and tensile properties of the novel metastable β titanium alloy Ti-5.5Cr-5Al-4Mo-3Nb-2Zr, a heat treatment process of ABFCA (α+β annealed with subsequent furnace cooling plus aging treatment) was designed, which can precipitate α phases of different sizes in the β matrix. The microstructure obtained by this heat treatment process is composed of a primary α (α_p) phase, submicro rod-like α (α_r) phase and secondary α (α_s) phase. The results show that the alloy with multiscale α phase has an excellent balance of strength-ductility. The elongation (EL) is ~18.3% at the ultimate tensile strength (UTS) of ~1125.4 MPa. In this experiment, the relationship between the strength of the alloy and the α phase is established. The strength of the alloy is proportional to the power of -1/2 of the α phase average spacing and width. The α_s phase with a smaller size and phase spacing can greatly improve the strength of the alloy by hindering dislocation slip. The transmission electron microscopy analysis shows that there is a large amount of dislocation accumulation on the α/β interfaces, and many deformation twins were found in the α_p phase after tensile deformation. When the dislocation slip is hindered, twins occur at the stress concentration, and twins can initiate some dislocations that are difficult to slip. Meanwhile, the plastic strain is distributed compatibly among the α_p, α_r, α_s phase and β matrix, thereby enhancing the ductility of the alloy. The ABFCA heat treatment process proposed in this study provides a reference for the heat treatment of metastable β titanium alloys, and this study has made a modest contribution to the development of titanium alloys.