Poor coherence of local oscillator signal restricts the azimuth/lateral resolution of long-range target imaging with synthetic aperture ladar (SAL). Coherence maintenance problem of SAL signal is studied, and a method for maintaining the coherence of SAL signal based on local oscillator digital delay is proposed. By setting the local oscillator signal reference channel and using the self-heterodyne detection method, the phase introduced by the frequency instability of the local oscillator signal is extracted in the digital domain, which is used to compensate the phase in target echo signal after the delay in the digital domain. The laser signal model is established, and the error analysis of the proposed SAL signal coherence maintenance method is given. The selection principle of the fiber delay length from the self-heterodyne detection is given. Theoretical analysis and simulation experiment show that the proposed method can effectively maintain the coherence of SAL signal.