In this paper, a method which can directly measure the thickness of compound thin films is given and the underlying principles are discussed. In the first approximation, the conclusion can be extended to the film with more layers. In order to increase the accuray of measurement, we obtained a more accurate equation and a set of data from electrinic computer. For the convenience of application, we set down all these data in the form of a handbook and express them with some diagrams.This direct method of measuring compound thin films on the substrate does not need any advanced equipment but only a ordinary ellipsometry. In principle, a PCSA optical system is to be used to solve the four-phase problem.