The invention provides a low-frequency broadband satellite-borne SAR imaging ionosphere phase error compensation method, which is characterized in that series decomposition is carried out on ionosphere phase error to three terms based on Legendre orthogonal basis, and the obtained errors are mutually orthogonal, so that the problem of order coupling of an existing assessment method is solved; then, based on an expression of each order derived based on an orthogonal model, influence of zero-order phase error on broadening of direction images, first-order phase error on translation of distance images, on pulse broadening caused by second-order phase error and on pulse distortion caused by third-order phase error are quantitatively evaluated; and based on the influence, ionosphere total electron content TEC information can be obtained, and ionosphere influence compensation can be carried out on the images. The method can accurately reflect the influence of true ionosphere phase error on low-frequency broadband satellite-borne SAR imaging, and thus ionosphere influence compensation precision is improved.