Composite strata have an important effect on the stability control of excavation face in tunnel engineering. Aiming at the deformation regularity of tunnel excavation face in composite strata, this paper analyzes the spatio-temporal evolution process of surrounding rock deformation through large scale similarity model test, digital image correlation (DIC) measurement method and medium-sized loading test system was conducted, and established mechanical model to explore the deformation and instability mechanism of the surrounding rock and excavation face of the tunnel with soft upper and hard lower. The research results show that: The spatial distribution characteristics of surrounding rock deformation of tunnel in the composite strata is that deformation first occur at above surrounding rock of tunnel, the arch produces settlement, the horizontal cracks in the interior and keep expanding, and the stress concentration occurs on the tunnel excavation face under load, which lead to collapse of excavation face and interior, and final instability failure of tunnel. Based on experimental observation, the model of tunnel surrounding rock in different failure stages is simplified to reveal the deformation and fracture evolution mechanism of tunnel excavation face in soft upper and hard lower strata. The strength of soft rock plays a key role in tunnel excavation in soft upper and hard lower strata. The fracture of soft rock excavation surface is greater than that of hard rock, and the settlement of arch roof is greater than that of arch bottom. Therefore, the soft rock part should be considered when choosing the support method.