Abstract: The global navigation satellite system (GNSS), besides being utilized for navigation and positioning services, also being regarded as signal source of opportunity for remote sensing of Earth surface environments and spatial targets through its advantages of high signal coverage and satellites visibility. Nevertheless, its inherent characteristics, including low power levels and limited bandwidth, pose considerable challenges to relative applications. The development of GNSS-based passive remote sensing system during the past three decades is reviewed, with a focus on the target detection technology. By analyzing the target detection processes of GNSS-based remote sensing system under both backward and forward scattering configurations, the key points and difficulties of relevant technologies are delineated. Progress and potential opportunities achieved domestically and internationally for addressing relative problems are also discussed.