Abstract: Based on a standard test method, steady-state tensile tests were carried out on SQ550FRW, a 550 MPa hot-rolled fire-resistant weathering steel sheet developed by Shougang Group at different temperatures from 20 to 1000 ℃. Typical mechanical properties such as modulus of elasticity, strength and elongation after fracture were obtained, and the corresponding equations of reduction factors were established. The results were compared with a cold-rolled fire-resistant weathering steel sheet called SQ420FRW and a hot-rolled fire-resistant thick plate called Q420FR. This research achieved the following six results. The SQ550FRW hot-rolled sheet exhibits exceptional mechanical properties at elevated temperatures. The elastic modulus decreases in two stages as the temperature increases, it shows a slow linear decline at 600 ℃ and below, and a rapid nonlinear decrease above 600 ℃. At 600 ℃, and the reduction coefficient of the elastic modulus is 0.704. The specified plastic elongation strengths, denoted as R_p0.2,T, R_p0.5,T, R_p1.0,T, R_p1.5,T and R_p2.0,T, gradually increase in that order. They remain stable above 520 MPa at 500 ℃ and below, and above 395 MPa at 600 ℃. The reduction coefficients for R_p0.2,T and R_p2.0,T are 0.723 and 0.912 respectively at 600 ℃. The tensile strength remains stable above 650 MPa at 500 ℃ and below, and is approximately 500 MPa at 600 ℃, with a reduction coefficient of 0.749. The nonlinear fitting models are established for the elastic modulus, for the specified plastic elongation strength, and for the tensile strength of SQ550FRW hot-rolled sheet as a function of temperature, with a goodness of fit exceeding 98.7%. Additionally, a Ramberg-Osgood constitutive model for stress-strain curves at different temperatures has been established, with a goodness of fit above 93.4%, which can provide data support for the structural design of fire-resistant weathering steel composite floor systems. Compared to SQ420FRW cold-rolled sheet, SQ550FRW hot-rolled sheet produced by MCCR (Multi-Mode Continuous Casting & Rolling Process) has excellent strength and high elongation greater than 16% at room temperature, indicating better formability; at the same time, it also has excellent high-temperature properties, and its reduction coefficients for specified plastic elongation strength and tensile strength both exceed 0.70, making it more suitable for manufacturing composite floor slabs.