Combined with the practical engineering of Changjiling tunnel in Guangdong highway, we analyzed the influence of different dimensions and positions of karst cave on deformation of the double track tunnel surrounding rock by FLAC3D. As the cave located above or below the left tunnel, the simulated result indicated the deformation characteristic of surrounding rock was similar. That was the Z-displacement took great part. And the displacements of the key points at the cross section of left tunnel decreased with increasing karst cave radius. But when the radius reached to a certain scale, the displacement also had enlargement trendy. On the contrary, the displacement of right tunnel′s key point increased with the increase of the cave radius, but the increased portion was minus to that of the left tunnel. When the cave located between left and right tunnel, horizontal displacement of the two tunnels surrounding rock increased and the Z-displacement decreased. And the displacement increased with the enlargement of size of karst cave. Meanwhile, deformation of the tunnel spandrel was asymmetric, that was, the displacement of tunnel closed to the caves was larger than the other.