Bi2FeMnO6 (BFMO) thin films with both conventional pseudo-cubic structure and novel supercell structure have been grown on SrTiO3 (001) substrates with different thicknesses of CeO2 buffer layers (ranging from 6.7 nm to 50.0 nm) using pulsed laser deposition. The correlation between the thickness of the CeO2 buffer layer and the structure of the BFMO films shows that the CeO2 buffer layer, as thin as 6.7 nm, is sufficient in triggering the novel BFMO supercell structure. This may be ascribed to the interfacial strain between the BFMO supercell structure and the CeO2 buffer layer which also serves as a seed layer. The buffer layer thickness is found to be critical to control the microstructure and magnetism of the formed BFMO supercell structures. Thin seed layers can produce a smoother interface between the BFMO film and the CeO2 buffer layer, and therefore better ferrimagnetic properties. Our results have demonstrated that strain and interface could be utilized to generate novel thin film structures and to tune the functionalities of the thin films.