<p>To optimize the formability of BR1500HS ultra-high-strength steel in automotive hot forming processes, uniaxial high-temperature tensile tests were conducted using a Gleeble−3500 thermal simulation testing machine. The rheological stress of this steel was investigated at temperatures ranging from 550 to 850 °C and strain rates from 0.01 to 10 s⁻¹. The results show that the rheological stress decreases with increasing temperature and increases with increasing strain rate. This is attributed to thermal activation and dynamic softening caused by rising temperatures, while increasing strain rates promote dislocation growth and suppress softening. Based on modified Zener–Hollomon parameters, a peak stress constitutive model was established, yielding a thermal deformation activation energy of 343.1916 kJ·mol^-1. A strain compensation mechanism was then introduced to construct a high-precision constitutive model, which achieved a correlation coefficient (<i>R</i>) of 0.9823 and an average absolute relative error (<i>AARE</i>) of 3.58%. Moreover, hot working diagrams under various strain conditions were plotted and analyzed, identifying the optimal processing window for BR1500HS steel as medium-temperature (700-800 °C) and medium-strain rate (0.5-2 s⁻¹). Under these conditions, the material exhibits good thermal forming stability, providing a theoretical basis for optimizing the hot forming process of BR1500HS automotive structural components.</p>