A Dy∶PGS laser with 4.3 μm high-power continuous laser generated by cascade oscillation is numerically simulated. The whole process of stable continuous output of Dy∶PGS laser generated by cascade oscillation is simulated, the laser power and the spatial distribution of population density in the resonant cavity are calculated, and the effects of pump power, crystal length and reflectance of output mirror on 4.3 μm laser output are analyzed. The calculation results show that the self-termination effect of Dy∶PGS crystal can be eliminated by cascade oscillation, and a 4.3 μm laser output with high power and efficiency can be realized. The output power of 4.3 μm laser can reach to 2.535 W with slope efficiency of 29% when the 1.7 μm pump power is 10 W. The optimal length range of the crystal is 12--24 mm, a higher reflectance of the idler outpot mirror is needed, and the optimal range of the reflectance of the signal output mirror reflectance is 0.8--0.9.