Because the target transmissivity (reflectivity) must be determined in advance in any conventional wideband monitoring of optical film thickness, this paper puts forward an imagination of mapping out parameters of film thickness and dispersive coefficients (refractive index) and continuously modifying target transmissivity in a coating time, by which the defect of the conventional wideband monitoring of optical film thickness can be overcome. The system of wideband monitoring of optical film thickness based on modifying target transmisivity has been built up. Its construction and working theory have been discussed. The accuracy of transmissivity measured by the system is higher than 2‰. A series of experiments of monitoring film thickness have been accomplished. It has shown that the accuracy of monitoring by the system is higher, the repeatability is better, and the accuracy of judging the end of coating is higher than1%.