Hot forming process of ultrahigh strength boron steel 22MnB5 is widely applied in vehicle industry. It is one of the most effective approaches for vehicle light weighting. Dynamic recovery is the major softening mechanism of the boron steel under austenite state at elevated temperatures. Deformation mechanism of the boron steel can be revealed by investigation on the behavior of dynamic recovery, which could also improve the accuracy of forming simulations for hot stamping. Uniaxial tensile experiments of the boron steel are carried out on the thermo-mechanical simulator Gleeble3800 at elevated temperatures. The true stress-strain curves and the relations between the work hardening rate and flow stress are obtained in different deformation conditions. The work hardening rate decreases linearly with increasing the flow stress. A flow stress model is derived based on Kocks model, and the derivative of the dislocation density with respect to the true strain, which is expressed by the peak stress and initial yield stress, is also deduced considering the dynamic recovery effect. A dynamic recovery efficiency factor h is defined as the ratio of the dynamic recovery effect and the dislocation accumulation effect. The effects of the deformation conditions on the h are analyzed.