As part of the untreated fillings of concealed Karst cave with internal pressure behind tunnel are complex and influenced by the seasonal precipitation and sudden precipitation, the increase of internal pressure can affect the stability of the structures near the tunnel, and may easily lead to the instability of tunnel primary support structure. Scaled model tests based on a 3-lanes highway tunnel were carried out to investigate the stability of tunnel excavation nearby Karst cave with internal pressure. Characteristics of the tunnel convergence, internal force of steel arch, surrounding rock pressure behind primary support and surrounding rock strain were analyzed to study the effect of position and pressure on the stability of the tunnel. The results show that, under the specific cavity size and spacing, tunnel lost stability firstly for Karst cave in the inverted arch. The convergence of the tunnel near the Karst cave increases with the increase of the pressure and changes suddenly when reaching the critical internal pressure, and the primary support loses its stability. The increase of pressure leads to the obvious increase of the bending moment of steel arch, and increases the axial force of steel arch evenly. Finally, the steel arch reaches the yield state, meanwhile, primary support becomes instability. Surrounding rock pressure behind the primary support near Karst cave increases obviously and leads to the uneven stress of primary support with the increase of Karst cave internal pressure.