Mixing efficiency is an important indicator of laser heterodyne detection systems that directly reflects sensitivity. On the basis of mixing efficiency theory combined with a partially Coherent Gaussian–Schell field model, the expression of mixing efficiency of partially coherent light is derived, and the relationship between spatial mismatch angle, spatial coherence length, receiving radius, and mixing efficiency is obtained. Results from the numerical analysis show that increasing the length of spatial coherence and receiving aperture can improve mixing efficiency but will lead to the reduction of the receiving field of view. When, the laser heterodyne detection system can obtain ideal mixing efficiency and field of view.