Mechanical properties of high-strength steel during the fire-heating and post-fire stages have been extensively investigated, while those during the fire-cooling stage were hardly studied. In this study, based on test data, a cooling stage influence coefficient (λ), namely the ratio of a mechanical property of steel in the cooling stage over that in the heating stage at identical elevated temperature, was specified to quantify the influence of the cooling process on the mechanical properties of steel. It was observed that when the peak heating temperature before cooling is within a certain value, the cooling stage influence coefficients of Young's modulus and strength were close to 1.0, while they were much less than 1.0 after the peak heating temperature exceeds this value. Besides, a series of formulas obtained by curve fitting of test results were proposed for quickly calculating λ of different mechanical properties for Q690 steel, including Young's modulus, yield strengths, ultimate strength, and ultimate strain. Moreover, a simplified constitutive model of Q690 steel was developed to present the stress-strain relationship of it, which is applicable to the fire-heating, fire-cooling and post-fire stages. The presented work enables more reliable structural analysis of high-strength steel structures throughout the whole heating-cooling process.