Influences of cooling rate on the microstructures, room temperature tensile properties, fracture toughness and impact properties of Ti55531 titanium alloy during β annealing were analyzed. The result shows that, Ti55531 titanium alloy after furnace cooling with different cooling rates, its microstructures are composed of straight grain boundary α phase (α_GB), residual β phase and intragranular lamellar α phase (α_WM). As the cooling rate is increased, the thickness of intragranular lamellar α phase decreases gradually, and the change of grain boundary α phase is not obvious. There has a direct relationship between the thickness of lamellar α phase precipitated during β annealing cooling and mechanical properties of Ti55531 titanium alloy. With the decrease of the thickness of lamellar α phase, the tensile strength and yield strength increase gradually, the elongation and reduction of area decrease gradually, and the fracture toughness and impact energy decrease gradually.