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 (solid solution in α+β region with subsequent furnace cooling followed by aging treatment finally) was designed, by which α phases of different sizes can be precipitated in the β matrix. The results show that the microstructure obtained by this heat treatment process is composed of primary α (α_p) phase, submicro rod-like α (α_r) phase and secondary α (α_s) phase. The alloy with multi-scale α phase has an excellent balance between strength and ductility. The elongation is about 18.3% at the ultimate tensile strength of 1125.4 MPa. The relationship between the strength of the alloy and the α phase was established. The strength of the alloy is proportional to the power of ?1/2 of the average spacing and width of α phase. 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 microscope analysis shows that there is a large amount of dislocation accumulation at the α/β interfaces, and many deformation twins are found in the α_p phase after tensile deformation. When the dislocation slip is hindered, twins occur at the stress concentration location, and twins can initiate some dislocations that are difficult to slip. Meanwhile, the plastic strain is distributed uniformly among the α_p, α_r, α_s phases and β matrix, thereby enhancing the ductility of the alloy.