By means of deformation and long term aging, the stability and phase equilibrim characteristic of the C+N synthetically strengthening austenitic Fe-Cr-Mn (W,V) alloy were investigated. Experimental results indicate that the austenitic alloy remains stability and no γ→α transformation occurs under 500℃. Synthetic addition of C and N causes the grains to refine and powerfully retards formation of ε martensite and precipitation of σ phase. M_s point is elevated with long term aging at elevated temperature (500-700℃) due to a large number of strain induced carbides precipitate. Along with accelerated decomposition of strain induced α' martensite and occurrence of recrystallization, γ→α transformation and σ phase precipitation are promoted so that austenite becomes unstable.