Abstract: Solid density is the main factor that represents the color quality of presswork,and it is determined by the ink thickness on the presswork.The thickness of the presswork is determined by the structure of the ink transferring system and the value of the ink key.Structure model of the ink transferring system of offset printing machine is established after the model of all the ink rollers in the structure is established.Then,the ink thickness equation is established based on the shortest ink transferring path of each ink roller contact point and ink transferring route of the system.By solving the equation,and according to the relationship between the ink thickness on the presswork and the ink thickness on the last ink roller,the relationship between the ink thickness on the presswork and the ink thickness on the ink fountain roller is obtained,i.e.,the ink transferring ratio.The relationship between the solid density and ink thickness under different conditions can be obtained by experiments.Through the calibration of the ratio of ink key value and the ink key opening size,the ink thickness on the ink fountain roller,the relationship between the ink key value and the solid density under different conditions can be obtained,i.e.,the ink-to-color control model.The model can be used in the color quality control.When the color quality shows the error,the ink key value can be adjusted to control the color based on the model.