Different from the homogeneous layer structure of famous Bin+1Fen-3Ti3O3n+3 compounds with integer n values, the mixed-layer structure of the compounds with fractional n values and their related physics have been rarely reported in recent years. In this work, the mixed-layer compound Bi11Fe3Ti6O33 (n = 4.5) was synthesized by the modified Pechini method, and its structure was characterized as an inhomogeneous phase generating from the disordered intergrowths of the n = 4 and 5 perovskite slabs. Multiferroic properties of this compound were discussed in detail, compared with two adjacent homologous Aurivillius phases Bi5FeTi3O15 (n = 4) and Bi6Fe2Ti3O18 (n = 5). Significantly, the ferroelectric polarization of the mixed-layer sample at room temperature is higher than that of the adjacent homologous phases with integer n, mainly arising from the intrinsic mixed-layer structure. The ferroelectric Curie temperature (similar to 992 K) and a magnetic transition temperature (similar to 7 K) of the 4.5-layer phase of Bi11Fe3Ti6O33 fall in between those of the homogeneous 4- and 5-layer phases, conforming to the change trends of such oxides with integer n. These results provide important contributions to understand the Aurivillius-type materials and open up a new avenue to enhance multiferroic properties in single-phase multiferroics.