The Sustainable Development Science Satellite (SDGSAT-1) was successfully launched on November 5, 2021. It is the world’s first scientific satellite dedicated to serving the United Nations 2030 Agenda for Sustainable Development. To evaluate the on-orbit image quality of its thermal infrared spectrometer (TIS), we propose an improved edge slope (ES) calculation method. Learning from past academic research, we found that the traditional ES calculation methods are limited by various factors. For example, it demands highly on geographical aspects and surface temperature uniformity. In this letter, we interpolate the narrow sea–land boundary region. The edge signal-to-noise ratio and the dip angle are calculated by the selected region to determine whether it satisfies the conditions of ES calculation. The Otsu algorithm is used to find the optimal threshold for the sea–land division, and the subpixel edge positions are extracted by combining with the canny operator. In order to achieve higher precision, we use the modified Fermi function to fit the extracted edge spread function (ESF). During the on-orbit testing of the SDGSAT-1/TIS, we performed ES calculations for a large number of test areas with a flight direction of 0.5 and a scan direction of 0.45.