javascript:void(0) This article deals with bifurcation dynamics and spectral entropy complexity of a nonlinear fractional-order SVIRS epidemic system. Based on the analysis of the disease-free and endemic equilibriums, the forward (supercritical) and backward (subcritical) bifurcations are observed around the critical point of R 0 = 1 , where R 0 represents the basic reproduction number of the system. The complexity of the fractional epidemic system is described by the spectral entropy, and the effect of the order of the fractional derivative on the system is analyzed with the complexity. Numerical simulations are carried out to support the theoretical analysis and to illustrate the bifurcation behaviors.