This paper studies testing technology of photoelectrical transmittivity, analyses the reason of error occurred in testing system, and find out the method for resolving it. It is high automation and accuracy than traditional methods. The system uses parallel light-pipe as lamp source, which is revised by correction filter, to obtain the white light of approaching visible spectrum trait. The white light is divided into two lights: reference light and measuring light. Reference light is coupled to modulator by optic fiber coupling device, modulator turns it into fixed frequency light and then it enters integral ball through ancillary lens. After measuring light passes by testing-piece, it is assembled to modulator by collector lens, modulator turns it into particular frequency light and then it enters integral ball through ancillary lens. The lights diffused reflection heaps of times in integral ball become the equality lights and are in proportion to flux of incidence. Electric eye converts light signal to electric signal. The electric signal contains two frequency signals, it is magnified by synchronous amplifier, and at last gains the electric signal in proportion to the light from pupil.