Abstract: The hot forging process rearranges and homogenizes the structure, which help improve the strength and toughness of titanium alloys. The fracture toughness and fracture mechanism of CT20 titanium alloy after hot forging at low temperature（20 ℃,-30 ℃ and-90 ℃） were studied. The dynamic evolution of crack tip plastic zone area at different low temperatures was analyzed by finite element method. The bending degree and length of crack growth path were quantified.The contribution of energy dissipation and crack growth to the fracture toughness of the alloy was determined. The results show that as the temperature decreases from 20 ℃ to-90 ℃, the energy required to consume the material during the fracture process decreases from 431.01 kJ/m~2 to 160.26 kJ/m~2, and the fracture mode gradually changes from ductile fracture to quasi-cleavage fracture. With the decrease of temperature, the plastic zone area of crack tip decreases, the stress concentration phenomenon is obvious, and the crack initiation resistance is weakened. The growth path changes from uneven to almost straight, the curvature decreases from 1.36 to 1.11, and the resistance of α phase boundary to crack growth is weakened at low temperature, which accelerates the crack growth.