The flow stress curves of Q345B microalloyed steel during hot compression deformation were obtained at 900-1100°C and strain rates of 0.01-10s-1 on Gleeble-3500 thermo-mechanical simulator. Based on the experimental results, a inverse regression analysis method was proposed to determine the mathematical model of dislocation density and its correlation coefficients using the flow stress curves, so as to laid the foundations for calculating carbonitrides precipitates, deformation resistance and the end-rolled strength of microalloyed steels during hot rolling precisely. The effects of the deformation parameters (strain rate and deformation temperature) and microstructural evolution (work-hardening, dynamic recovery and dynamic recrystallization) on dislocation density were taken into account. Finally, the validity of the proposed model is discussed considering the plausibility of the parameters assessed for evaluating the dislocation density. It was proved that the model was reasonable and it could provide a guide to production.