In this papers,we interpreted the basic theory of using the UV visible Spectrophotometer method to test the optical properties of Cadmium Sulfide(CdS)films,and designed the testing system.The rate of optical transparency of CdS films from the Chemical Bath Deposition have been tested by the system we designed.The properties of CdS films which deposited with various time have been analyzed.According to it,we drew an conclusion that in the same conditions the time of deposition have greatly effected on the optical properties of CdS films.