The invention discloses a standard interference piece fitting method capable of acquiring film thickness and refractivity simultaneously. A to-be-detected film only needs to be coated on two standard interference pieces coated with a layer of film with already-formed interference and different refractivity. Thus, through simultaneously fitting a transmitting (reflecting) spectrum before and after coating of the two standard interference pieces, the thickness and the refractivity of the to-be-detected film can be acquired precisely at the same time, the film thickness measurement limit can be as high as 1nm, and compared with the 30nm measurement limit in the traditional method, great improvement exists. In addition, as the two standard interference pieces are fit simultaneously, errors of a test system can be greatly reduced, and requirements on performance of the test system and the cost can be significantly reduced. The method is applicable to online detection and monitoring of a film coating system and can be promoted and used in industrial production or scientific research.