A kind of SEIRS epidemic model with valid term is investigated． By choosing the effective contactrate as the parameter，the existence of the Hopf bifurcation is analyzed，and the phenomenon that the positive equi-librium is still keeping stable when the effective contact rate β is small but the stability will be destroyed while theeffective contact rate processes a critical point and Hopf bifurcation appears at this point is indicated． Finally，theexplicit formulates to determine the stability and direction of bifurcating period solutions are deduced by using thecenter manifold and the normal form theories．