In a synthetic aperture imaging ladar (SAIL), the imaging quality is greatly degraded from the laser speckle effect of diffused target. A systematic solution to reduce speckle is given, and thus the theoretic basis is founded for the design of SAIL structure and operating mode. The speckle characteristics relating to the scale of target resolution element, the wavelength change from laser chirp, the correlation of target surface and the uncertain distribution of intensity at receiver plane are investigated. A complex coherence function of integrated speckle over antenna aperture is defined, it is a convolution between the correlation of antenna profile and the complex correlation factor of the speckle from a resolution element and its width is the possible length for aperture synthesizing. A design principle for transmitter aperture, receiver aperture and used length for synthesizing is given to achieve a long synthesizing length. And a fluctuation phenomenon of beat signal due to the speckle wandering caused by the chirp is also discovered and analyzed. Then a sliding spotlight mode which has a magnification from the SAIL movement to the beam scan is considered to effectively utilize the limited length of aperture synthesizing. A multi-channel transmitter/receiver construction is proposed to enhance the detectability of speckled echo.