Abstract: To further improve the high temperature service performance of 5 CrNiMoVNb steel such as the high temperature thermal stability and thermal fatigue performance, the microstructure was analyzed by TEM and the equilibrium precipitates were calculated by thermodynamic calculation software. The effect of alloy elements such as Cr, Mo, V on the content, thermal stability temperature and coarsening rate of the main precipitation strengthening phases was analyzed. The results show that the precipitation strengthening phases in 5 CrNiMoVNb steel are mainly M₂₃C₆ and MC＿ETA below 700 ℃,most of the carbide particles are dispersed on the martensite matrix. The content, thermal stability temperature and coarsening rate of M₂₃C₆, as an important grain boundary strengthening phase, is mainly dependent on the content of Cr and V. After comprehensively analyzing the effect of alloying elements on the content, thermal stability temperature and coarsening rate coefficient of the main precipitation strengthening phases, the main alloy composition（wt%） of 5 CrNiMoVNb steel is optimized as follows:1.2-1.5 Cr, 2.2-2.4 Mo, 0.6-1.0 V.