The general transmission formula of F-P etalon including different reflectivity and absorption is derived. When the reflectivities of the two surfaces are the same and the absorption is ignored, the general formula will become the common formula in the documents or books. When the reflectivities are different and the absorption is ignored, the maximum transmission is not 100%, but there is a certain reflectivity. Moreover if the difference between the reflectivities of the two surfaces is bigger, the effect becomes more obvious.