In order to find the influence of α phase on tensile fracture behavior of high strength-toughness Ti-55531 alloy, different heat treatments and analysis methods including SEM, OM and XRD are adopted. Results show that the content of secondary α phase in bimodal microstructures are increased and the scale became finer, with decreasing of content and scale of primary α phase, so that the strength of alloy are increased and ductility are decreased. In lamellar microstructures, with coarsening of secondary alpha phase, both strength and ductility of the alloy are decreased. In full β microstructures, alloys get well in ductility but bad in strength. Both fracture modes of bimodal and lamellar microstructures are mix modes including transgranular cleavage, microvoid accumulations and intergranular crack fracture. As decreasing of content and scale of primary α phase, the content of secondary α phase are increased and the scale are coarsen, the percentage of microvoid accumulations fracture are decreased and percentage of transgranular cleavage and intergranular crack fracture are increased.