Starting from nonlinear equations on the 〖WTB1X〗f〖WTB1〗plane containing frictional dissipation and condensation heating under the Boussinesq approximation,the equation of generalized energy in moist air is presented,and the necessary condition of subcritical moist symmetric instability is derived.The influence of condensation heating on the subcritical symmetric instability is derived analytically at first.Then the critical values of the initial disturbance amplitude are calculated in the cases of unstable and stable mesoscale atmosphere stratification.It is found that subcritical symmetric instability is easier to happen in moist saturated air with either the unstable or stable stratification than in the dry air,and easier with the unstable than stable stratification.The results improve the theory of nonlinear subcritical symmetric instability in the dry air.