The compression test was conducted on 9Cr18Mo stainless steel by using the UTM5305 universal testing machine and the split Hopkinson pressure bar (SHPB) test device. In this way, the stress–strain curves pertaining to quasi-static (strain rate of 0.001 ~ 0.1 s-1) and dynamic (temperature range of 25 ~ 650 ℃ and strain rate of 800 ~ 4,000 s-1) states were attained. According to the stress–strain curves, the rheological behaviours of 9Cr18Mo stainless steel at high temperature and high strain rate were discussed. Based on the test data, the parameters of two constitutive models (Johnson-Cook (J-C) and Power-Law (P-L)) for 9Cr18Mo stainless steel were identified and the correlation coefficients (R) and average absolute relative errors (AAREs) of the two constitutive models were compared. The results showed that 9Cr18Mo stainless steel presents strain-rate sensitivity and significant thermal softening, that is, the flow stress on 9Cr18Mo stainless steel increases with strain rate while significantly reduces with increasing temperature. The R values are 0.9697 and 0.9896 and the AAREs of two constitutive models are 2.77% and 1.85%, respectively. Hence, the P-L constitutive model shows a higher prediction accuracy compared with the J-C constitutive model and can better describe the rheological behaviours of 9Cr18Mo stainless steel at high temperature and high strain rate.