Abstract: GNSS radio occultation technique has been demonstrated to be a powerful tool for remote sensing of the Earth's neutral atmosphere and ionosphere.It receives GPS signal through a dual-frequency GPS receiver onboard LEO satellites.Affected by the vertical refractive index of the communication medium,the propagating path will be curved,which would result in a time delay when arriving at the receiver.From this time delay,we could obtain the ionospheric refractive index.Then the ionospheric electron density profiles could be derived.Relying on its characteristics of all-weather,high vertical resolution and broad coverage,which other ionospheric surveying methods do not have,the radio occultation method has laid foundation for the new ionospheric remote sensing technology,and offered a unique chance for improving our knowledge of ionospheric behavior.Firstly,we review different inversion methods and application examples of GNSS ionospheric radio occultation.Finally,we investigate and summarize the existing problems as well as its future research directions.