To maintain the stability of excavation face in slurry shield, a filter cake must be timely formed during the excavation work. With the filter cake, the slurry pressure can counteract the earth pressure and water pressure in the excavation stratum. In this study, a mathematical model of slurry penetration under constant pressure in sand stratum is established by considering the filtration effect. By means of this model, the influence of penetration time, slurry concentration, slurry pressure and initial stratum porosity on soil porosity and filter cake formation is analyzed. Combining the periodic disturbance of cutting-tools, the variation of soil porosity on the excavation face caused by multiple slurry penetration is summarized. Finally, the supporting mechanism of the excavation face is discussed during the shield tunneling based on the design of cutting wheel and tunneling parameters. The study results show that the slurry penetration behaviour is mainly affected by the characteristics of slurry stratum without disturbance of cutting-tools. In contrast, with the periodic disturbance of cutting-tools, the decreasing cycle of cutting will increase the difficulty of filter cake formation under the same cutting depth. In addition, the filter cake formation and the stability of excavation face can be significantly improved if the number of cutting-tools on same cutting track is reduced and the advance speed of shield and rotate speed of cutting wheel are decreased appropriately.