An ultrafine-grained (UFG) pure zirconium(Zr) refined by compounding with a size of Φ4 × 6 mm was subjected to a unidirectional compression test using a Gleeble-3800 thermal simulation tester at the temperature of 300 °C–450 °C and a strain rate range of 0.001–0.05 s_-1. Experimental results show that the process parameters influence the flow stress of the UFG pure zirconium. Experimental data and microstructure analysis, reveal that DRX is more likely to occur at higher deformation temperature and lower strain rate. The critical strain and critical stress of UFG pure zirconium were determined. In addition, a critical strain model of UFG pure zirconium was established by introducing Zener–Hollomon paramete Z. The constitutive model of UFG pure zirconium was confirmed based on DRX volume fraction and proven to occur at a strain of 0.1 to 0.45.