Lanthanum doped bismuth layer-structured ferroelectrics and its intergrowth type, SrBi4-xLaxTi4O15, Sr2LaxBi4-xTi5O18, and (Bi,La)4Ti3O12-Sr(Bi,La)4Ti4O15, have been prepared by solid-state reaction method. The dielectric properties of those materials are measured. The results show that SrBi4-xLaxTi4O15 (x=1.0), Sr2LaxBi4-xTi5O18(x≥0.5)， (Bi,La)4Ti3O12-Sr(Bi,La)4Ti4O15(x=1.50) exhibit typical relaxational characteristics. The degree of diffuseness and relaxation increased with increasing La3+ content. The origin of relaxation is the microdomain induced by La3+ doing. The micromechanism corresponding to the transition between normal ferroelectrics and relaxors is explained as the transition from the macrodomain of normal ferroelectrics to the microdomain of relaxors.