Abstract: In complex application scenarios, the velocity of radar platform cannot be kept constant. The existence of acceleration increases the coupling of range and azimuth of bistatic SAR, which makes the imaging processing more difficult. In this paper, an orthogonal decoupling method of two-dimensional spectrum is proposed to solve the imaging problem of bistatic SAR under acceleration. Firstly, Chebyshev polynomials are used to expand the slant range into higher-order power series, which improves the accuracy of slant range approximation. Then, the method of series inversion is used to obtain the high-order approximate two-dimensional spectrum of echo signal. On this basis, the coupling phase terms of the two-dimensional spectrum are decomposed by Chebyshev orthogonal polynomials to eliminate the range-azimuth coupling. Finally, focus imaging is realized by phase compensation. Experimental results show that this method can effectively overcome the influence of acceleration and improve the imaging quality of point targets. Theoretical derivation and simulation results verify the effectiveness and feasibility of the algorithm.