In this paper, TC21 titanium alloy with high strength and toughness was selected, and the heat treatment process of "Step quenching" was used to regulate and optimize the multi-scale lamellar microstructure and its mechanical properties. The microstructure morphology, fracture morphology and cross-section crack propagation morphology of multi-scale lamellar microstructure were investigated by SEM and TEM. The results show that the isothermal quenching temperature has a strong influence on the α phase precipitation behavior and the mechanical properties of the alloy. The samples were solution treated at 930 ℃ for 1h, and then step-quenched with temperature from 0 ℃ to 600 ℃. With the increasing of temperature, the width of secondary α lath phase gradually increased, and the hardness first increased and then decreased slightly, among which the hardness at 400 ℃ being the highest. The samples were solution treated from 880 ℃ to 960 ℃ for 1h and aged at 400 ℃ for 2h with water cooling to room temperature. Thus, coarse lamellar, multi-scale lamellar and fine lamellar microstructure were obtained respectively, and the hardness and strength of the alloy increased successively. However, due to its tortuous crack propagation path and crack deflection characteristics, the multi-scale lamellar microstructure showed excellent crack propagation resistance (KQ=104 MPa?m1/2) which was significantly higher than the coarse lamellar (KQ=67 MPa?m1/2) and the fine lamellar microstructure (KQ=33 MPa?m1/2).