The printing pressure of ink droplets in inkjet printing comes from the droplets kinetic energy after jetting, the energy of the ink droplets infiltrating and diffusing in the substrate is derived from the conversion of ink droplets kinetic energy, this is one of the main factors of producing the dot gain phenomenon in the inkjet printing. The mathematical model of ink droplets infiltrating in the substrate was researched in this paper, we analyzed the different structures of paper coatings at first, proved that the paper substrate was the porous medium, so the process of inkjet imaging was the process of droplets infiltrating the porous substrate and drying. But traditional model is no easy to use in practice, we studied this problem based on the energy dissipation and conversion view and obtained the new mathematical model. At the same time, the imaging results of the ink droplets in the paper coating were analyzed in this paper by QEA image analysis instrumentation, the experimental results showed that the imaging process of ink droplets in the substrate could be represented by the mathematical model of the discrete-phase fluid flowing in the porous media, these results described the causes of the inkjet imaging dot gain and provided a scientific theory in order to solve the dot gain problem in the inkjet imaging printing system.