A high-energy laser system with self-sustaining adaptive optics is designed, which uses active illumination detection method of high-energy laser to detect target. However, the echo field received is affected by speckles, and a physical model of the detection system is established to analyze speckle effect. The average scale of speckle is discussed based on the autocorrelation function of speckle intensity. The complex coherence function of speckles on the receiving plane is defined via the partial coherence theory, and the width of complex coherence function in the integral field with different scales of speckle patterns is discussed as well. The imaging magnification of target and the ideal resolution are derived, and the relationship between the image scale and the light spot scale on the focal plane is discussed. The effect of the spot array on the focal plane with different scales of speckles is analyzed by numerical simulation. By superimposing atmospheric turbulence, the effect of speckle field on the atmospheric turbulence detection is confirmed. The results show that the speckle scale between subaperture and large aperture is the best design, when the system has both high displacement measurement precision and high overall detection rate.