Abstract: The ionosphere photometer (IPM) onboard China’s second-generation, polar-orbiting meteorological satellite Fengyun-3D is the first all-weather quantitative optical remote sensing payload in China for the space-based surveillance of the ionospheric environment, capable of detecting the ionospheric airglow intensity of the nighttime/daytime OI 135.6 nm and the airglow intensity of the daytime N₂LBH. The key ionospheric environmental parameters, including the total electron content (TEC) at night, the F2 layer peak electron density (NmF2), and the daytime O/N₂ ratio, can be deduced. The payload enjoys the outstanding advantage of high sensitivity at night, suitable for the detection of the fine structure and the small changes of the ionosphere. It is built in with two operation modes for the night and day observations, respectively, to realize all-weather monitoring of the ionosphere. This paper analyzes the detection goal, the remote sensing principle, the system composition and function, as well as the in-orbit detection results of the IPM.