Band integral transmission was defined and plastic film thickness measurement model was built by analyzing the intensity variation when the light passes plastic film, after the concept of band Lambert Law was proposed. Polypropylene film samples with different thickness were taken as the research object, and their spectral transmission was measured by the spectrometer. The relationship between thickness and band integral transmission is fitted using the model mentioned before. The feasibility of developing new broad band plastic film thickness on-line measurement system based on this method was analysed employing the ideal blackbody at temperature of 500 K. The experimental results indicate that plastic film thickness will be measured accurately by integral spectrum method. Plastic film thickness on-line measurement system based on this method will hopefully solve the problems of that based on dual monochromatic light contrast method, such as low accuracy, poor universality and so on.