The invention provides an in-orbit modal identification method for a synthetic aperture radar (SAR antenna) of a satellite. The in-orbit modal identification method comprises the steps of: acquiring an acceleration response signal generated by the SAR antenna excited by pulses during satellite in-orbit steady-state operation through a satellite in-orbit vibration monitoring and modal testing system; and performing modal identification on the acceleration response signal by adopting a random subspace identification method, thereby determining modal frequency components of the SAR antenna in the in-orbit state. The in-orbit modal identification method for the synthetic aperture radar of the satellite solves the problems of inaccurate test of modal frequency and low resolution of the modal frequency of the SAR antenna on the ground, and ensures the accuracy degree of satellite in-orbit modal analysis.