The invention provides an onboard synthetic aperture radar movable target detection external field test method, which can realize equivalence of high orbit synthetic aperture radar, so that onboard synthetic aperture radar can enroll movable target echo data with long synthetic aperture time, to provide support for verifying the feasibility of a high orbit synthetic aperture radar movable target detection technology. The onboard synthetic aperture radar movable target detection external field test method is applicable to the imaging test of a ship target, and is also applicable to the imagingtest of time-sensitive target such as water body, crops and forest in long synthetic aperture time.