A metastable β titanium alloy Ti-5.5Cr-5Al-4Mo-3Nb-2Zr was designed. Three types of microstructure with different α phases were obtained by different heat treatment methods. The effects of α phase on stress corrosion behavior of the alloy were investigated by SEM, TEM, electrochemical test, and slow strain rate tensile test. The results show that after aging treatment at 650 ℃ for 6 h, the secondary α_s phase is coarsened (microstructure No.: M1). After solution treatment at 790 ℃ for 0.5 h at the two-phase zone, a coarse primary α_p phase (microstructure No.: M2) is formed. After solution treatment at 790 ℃ for 0.5 h and aging treatment at 650 ℃ for 6 h, coarse primary α_p phase and delicate secondary α_s phase (microstructure No.: M3) exist simultaneously. The electrochemical test results show that the self-corrosion current density of M3 is relatively low of 1.10×10^-8 A/cm². The polarization resistance of M3 is the highest at 2.30×10¹⁰ Ω·cm², which indicates the best corrosion resistance of M3, followed by M2, and M1 has the worst corrosion resistance. The results of slow strain rate tensile tests indicate that M3 has the lowest stress corrosion susceptibility index of 5.0%, and its stress corrosion cracking susceptibility is relatively low. The interaction of the hydrogen absorption-induced dislocation emission and the hydrogen-enhanced localized plasticity can explain the stress corrosion cracking mechanism.