This paper firstly discusses the properties of modified asphalt by blending petroleum asphalt and coal tar pitch. Then, a new mathematical model which was called ZQL was constructed. Based on three variables, property, granularity size, and proportion, the three-dimension function was fit, where the property was set as a dependent variable and the rest of the variables were set as independent variables. The different functions were finally obtained. Finally, based on the British standard of road asphalt, we solve the ZQL model and obtain a suitable result which matches the experimental results very well.