Micro emulsion- hydrothermal method was used in preparing a series of three- layer PI compositefilms，which have different thickness ratio of doped PI layer and pure PI layer． Their microstructure were tested byTEM and the dielectric properties were measured respectively by test of breakdown strength and electric corona，inthe same condition． The dimension scale of composite films was clearly shown from TEM test，and nano- particlesdispersed well in organic matrix． It can be observed that the breakdown strength is decreased，and electric coronahas a trend of increasing and then decreasing with the increase of thickness ratio of doped PI layer．