Abstract: The heat treatment process was designed to regulate the size and morphology of α phase of TC4 ELI titanium alloy. The tensile and impact tests at room temperature were carried out. The microstructure morphology of the alloy and the microstructure characteristics of the longitudinal section of the crack propagation region of the impact sample were observed by OM. The braided multi-scale lamellar microstructure size was determined by quantitative metallographic method. The fracture morphology characteristics of the crack propagation region were observed by SEM. The results show that the content and size of lamellar α phase in secondary phase can significantly affect the mechanical properties of TC4 ELI titanium alloy.The aspect ratios of α phase obtained by four heat treatments are 10∶1,15∶1,40∶1 and 400∶1, respectively. The strength of the alloy increases first and then decreases with the increase of the first heat treatment temperature, while the plasticity decreases continuously. The braided multi-scale lamellar microstructure obtained by the sample after 965 ℃×1 h/AC+680 ℃×2 h/AC treatment shows excellent comprehensive mechanical properties. The tensile strength reaches 949 MPa, the yield strength reaches 828 MPa, the impact toughness reaches 68.5 J/cm~2, and the fracture morphology is dimple. The mechanism analysis shows that this is due to the lamellar α phase in β matrix has appropriate aspect ratio, dislocation slip is difficult to penetrate the crystal interface, tortuous crack deflection and a large number of secondary cracks.