In order to obtain the unknown motion target imaging under low SNR in the LFM system, a new accumulation imaging method based on frequency modulated Fourier transform was proposed. Based on the characteristics of the target's LFM radar interpulse echo signals, the target's echo signals were jointly matched and accumulated in fast-slow time domain. Through the target position after energy accumulation, the target's accurate motion parameters could be searched out. After that, the compensation function was constructed to compensate the target motion, so as to obtain the accurate ISAR image. Computer simulation results showed that the proposed method could be robust under low SNR.