For the sake of solving the color mark detecting problem of overprinting error information in gravure printing, an optical module of detecting color marks is designed and realized based on the principle that the reflected light energy absorption is different when various colored bars are irradiated by a white light source. In addition, a sort of glass slice with one face possessing semitransparent capacity, the half reflectivity and the function of a photocell’s photovoltaic conversion are utilized. As a result of weakness and non-digital of output signal, an output signal is magnified and transformed. The experiment result shows that the sensor has many good features, such as an uncomplicated structure, far detecting distance((10±2) mm), little facula radius(Φ1.5 mm), high sensitivity, fast response speed(0.2 ms), high stability and reliability, etc.