Many functional properties of perovskite oxides, ABO(3), have a close relationship with their subtle structures, especially, the distortions of BO6 octahedra. As perovskite oxide heterostructures offer a good platform for controlling functional properties, it is important to understand the coupling between strain and oxygen octahedral distortions across the interface. Here, we study the coupling between the interfacial strain and oxygen octahedral distortions in GdScO3/SrTiO3 heterostructure using aberration-corrected transmission electron microscopy (AC-TEM) with sub-Angstrom resolution and complementary first-principle calculation. Our results reveal that the shape rather than tilting angle of octahedra is strongly modified by the interfacial strain. These results would conduct us to deeply understand the strain-octahedral distortion coupling mechanism, and tune functionalities of perovskite oxide thin film in a more elaborate way which is related to the geometry of cation-oxygen bond.