Reflectance prediction model for variable dot-size ink jet presswork was studied based on Clapper-Yule model. At first, variable drop size and the FM screening form of Clapper-Yule model were introduced; then, Clapper-Yule model was extended using the change of ink film thickness and FM screening dot, which were caused by drop change; at last, the extended Clapper-Yule model with ink drop size was established. The advantage of the model was validated with simulation.