Abstract: Compared with the CoCrNi medium-entropy alloy (MEA), the CoCrNiSi0.3 MEA exhibits more excellent synergistic mechanical behavior of strength and toughness under quasi-static loading. This is mainly attributed to the addition of an appropriate amount of Si element, which reduces the stacking fault energy of the alloy. This promotes the formation of deformation twins with smaller thickness and higher density during plastic deformation of the alloy, and an FCC to HCP phase transformation occurs. Temperature and strain rate are important influencing factors of material properties and are of great significance for understanding the strength, toughness and workability of materials. In this paper, the dynamic compression experiment at room temperature (20 ℃) and the high-temperature quasi-static compression experiment of CrCoNiSi0.3 MEA were carried out by the Hopkinson pressure bar. The mechanical behavior and deformation mechanism of CoCrNiSi0.3 MEA with strain rate and temperature dependence were systematically studied. And based on the experimental data, a modified Johnson-Cook constitutive model was established to predict the mechanical behavior of CoCrNiSi0.3 MEA well. The results show that under dynamic loading, the yield strength of the alloy increases with the increase of the strain rate, and the average work hardening rate increases slightly at first with the increase of the strain rate. However, when the strain rate reaches about 5196 /s, due to the formation of shear bands, the average work hardening rate decreases. Compared with quasi-static loading, CoCrNiSi0.3 MEA shows a higher strain rate sensitivity under dynamic loading. Under the strain rate of 5196 /s during dynamic compression, shear bands, high-density stacking faults, deformation twins and nanoscale HCP phases are found. The combined action of these mechanisms provides higher yield stress for the material. Under quasi-static loading, with the increase of temperature, the yield stress and work hardening ability of the alloy decrease significantly. When the temperature reaches 1000 ℃, the material presents an ideal elastic-plastic mode and does not show work softening. In particular, at 600 ℃, the yield strength and flow stress of the alloy are basically the same as those at 400 ℃. Whether at room temperature or at a high temperature of 600 ℃, nanocrystalline regions caused by local deformation are observed in the deformed samples, and the width of the nanocrystalline region is larger and the grain refinement is more complete at high temperature.