This proceedings volume contains selected and expanded contributions presented at the 6th International Symposium of Space Optical Instruments and Applications, held in Delft, the Netherlands on Sep 24th-25th, 2019. The meeting was organized by the Sino-Holland Space Optical Instruments Joint Laboratory and supported by TU Delft. The symposium focused on key innovations of space-based optical instruments and applications, and the newest developments in theory, technology and applications in optics, in both China and Europe. It thus provided a platform for exchanges on the latest research and current and planned optical missions. The major topics covered in these conference proceedings are: space optical remote sensing system design; advanced optical system design and manufacturing; remote sensor calibration and measurement; remote sensing data processing and information retrieval; and remote sensing data applications.