The effect of laser speckle on heterodyne detection for synthetic aperture imaging ladar (SAIL) is investigated. The smooth effect due to the finite aperture of SAIL antenna is considered, and particularly the statistical characteristics of the time-varying speckle pattern is explored and mathematically analyzed, which is caused from the frequency modulation of laser within a single look. As a result, the signal-to-noise ratio of photon-limited heterodyne detection due to the space-time speckle effect is given.