Abstract: A positioning method in footprint of space-borne laser altimeter based on vertical line locus (VLL) algorithm and elevation structure constraint is proposed. In the space-borne laser altimeter supported satellite photogrammetry frame, positioning point candidates are acquired in the footprint with digital surface model(DSM) generated by stereo images, according to the relative elevation structure information of the waveform. Then, elevations of positioning point candidates are also improved using VLL algorithm compared with the elevation structure information, which can eliminate the position errors of stereo images. Experiments show that the method proposed is effective in big footprint space-borne laser altimeter positioning, and the elevation positioning accuracy is 0.16 m, plane positioning accuracy is the same with stereo images.