Two-dimensional planar heterojunctions composed of single-layer transition metal dichalcogenides have great potential for the fabrication of low-power, high-performance, and flexible optoelectronic devices. The localized atomic structure and crystal defects at interface govern the electronic, magnetic, optical, catalytic, and topological quantum properties. However, precise characterization of interface atomic structure is still a challenge, so far. To determine the accurate atomic position, a spherical aberration-corrected electron microscope with segmented detector is employed, and the calculation by Integrated Differential Phase Contrast (iDPC) imaging algorithm has been performed. By using iDPC method, we have carried out characterization for the atomic structure of WS₂-MoSe₂ monolayer heterojunction interface, while imaging the W, Se, Mo, and S atoms simultaneously. Statistics show that the distribution of angle between the lattices on both sides of the WS₂-MoSe₂ planar heterojunction is around 29° and 35°. Additionally, we found that the lattice near the boundary experiences strains of approximately 4‰ and 2% in the two lattice vector directions, with significant distortion occurring only at the interface. In this work, several typical atomic configurations, including merge type, quadrilateral type, and pentagonal type, are found. The interface atomic configurations could help to release stress at the lateral interface. This study provides a useful method for the accurate structural characterization for planar heterojunctions of monolayer transition metal dichalcogenide. It is of great significance for an in-depth investigation of structure-property relationships at single-atom resolution in various interface structures.