The invention discloses an onboard InSAR data processing method based on a BP algorithm and a time-varying baseline, and belongs to the field of radar imaging and InSAR signal processing. According to the onboard InSAR data processing method based on the BP algorithm and the time-varying baseline, the BP imaging algorithm is used for obtaining complex image data of double antennas, an interferometric phase representing the landform elevation is obtained through phase difference processing, phase filtering processing and phase unwrapping processing, and an accurate digital landform elevation is obtained through an elevation inversion method of the time-varying baseline. According to the BP imaging algorithm, an imaging area is divided into grids, and coherent accumulation in the azimuth direction is carried out on data obtained through pulse compression processing in the distance direction pulse by pulse to obtain high-accuracy complex image data according to motion tracks of phase centers of the antennas. According to the elevation inversion method of the time-varying baseline, parameters of the time-varying baseline change along the imaging grids in the azimuth direction point by point, namely, the parameters of the time-varying baseline change along the slow time, and the parameters of the time-varying baseline are determined by the space positions, at each moment, of the phase centers of the two antennas, and comprise the length of the time-varying baseline and the dip angle of the time-varying baseline.