In order to evaluate the performance evolution of high-strength steel bars after fire, this paper studied the effects of air cooling, furnace cooling and water cooling on the stress-strain curves of high-strength steel bars after high temperature, and the variations of mechanical parameters such as yield strength, ultimate strength, strength yield ratio, elastic modulus and elongation after fracture are analyzed. The test results showed that: 1) The stress-strain curves of high-strength steel bars still showed obvious yield steps under the action of air cooling, furnace cooling and water cooling after high temperature; 2) Compared with normal temperature, both the ultimate strength and yield strength showed a decreasing trend, but the strength yield ratio did not change significantly; 3) Different cooling method has different effects on the elastic modulus and elongation after fracture of high-strength steel bars, in which water cooling had the greatest effect on the elastic modulus of steel bars, while furnace cooling had the greatest effect on the elongation after fracture of steel bars. Additionally, the tensile constitutive relationship of high-strength steel bars under different cooling methods is also established, which provided a basis for the application of 600 MPa high-strength steel bars in the design of fire-resistant structures.