The missile-borne synthetic aperture radar (SAR) often use nonlinear scaling algorithm to improve the focusing effect. However, the nonlinear scaling algorithm cannot be used in missile-borne spotlight SAR because of its own limitations. Aiming at this problem, this paper first puts forward a new method named “sub-band compensation” and combineds it to the missile-borne SAR spotlight imaging algorithm. This new algorithm does not introduce the three phases of operation, so the Doppler frequency rate can be effectively compensated under low PRF. Simulation results and analysis show that compared with the nonlinear chirp scaling algorithm, the proposed one has advantages in computation, results obtained, and applicability.