Abstract: Considering the case in which the total population is nonconstant and the lifelong immunity of population is absent, a kind of SIRS model with standard incidence is proposed. The basic reproduction number R₀ is obtained by a renewal equation. Moreover, the global stability of equilibria is proved by a suitable Lyapunov function. The results show that if R₀<1, then the disease-free equilibrium is globally asymptotically stable; if R₀>1 and the loss rate of immunity is large enough, then the endemic equilibrium is globally stable.