Polymer films thickness is one of the critical factors for the operation performance of the integrated optical devices (IODs) based on polymer planar waveguides and it is difficult to real-time determine it during corresponding fabrication process. A variety of approaches, which have been presented before, are either of less precision or more contamination of the film surfaces. In this paper a practical method, in which the absorbance of the polymer film using Lanbert law to determine the thickness of spin-coated polymer films, was presented. Compared with other methods, it can get an acceptable accuracy without using specialized and expensive instruments, and the surface of the film with not be contaminated during the measurement process. The experimental results compared with other methods were listed and depicted in confirming our method's availability. The factors that may have influence on the employment of our method were analyzed and discussed.