Generally, a safe margin equation is established by critical load coefficient and external load coefficient of the system in structural integrity stable reliability analysis, and then the reliability index of structural integrity stability is educed. Critical load coefficient is always obtained on condition that structure is acted on by one load. Actually, besides one load acting on the structure, their distribution is not always the same. In addition, because of the stochastic characteristic of structure system itself (e.g., cross-section of element, length, modulus of elasticity etc. are random variables), it is difficult for structural integrity stable reliability analysis. According to the theory of structure system stability and basal concept of reliability, the safe margins of structure system under different distribution loads are presented in this paper. The expression of safe margin sensitivity is given by stochastic finite element method because the safe margin is the expression of implicit function. Furthermore, the reliability indexes of the safe margin under different condition are educed by the advanced first order second moment method (AFOSM). By comparison and analysis, the structural integrity stable reliability index is ultimately obtained.