Abstract: The excavation surface model of the composition geologic strata for the shield machine was established, in which the soft soil and hard rocks are taken into account simultaneously. In terms of the cutting mechanism of the individual disc cutter and drag scraper, the total bending moment model on the cutterheads of shield machines was proposed. The bending moment behavior was discussed when the shield machine excavates in the composition geologic strata. The results show that the bending moments at x and y directions may be described by the cosine and sinusoid curves, respectively. The amplitude of the bending moment decreases with the increase of the fraction of soft soil on the excavation face. The average moment is maximal when the ratio of the soft soil and hard rock is about 50 percent.