Coherent detection mixer is an essential component of spatial coherent light communication system. Its main function is to mix signal and local oscillator lights to obtain an intermediate frequency signal and then enlarge the optical signal so that the sensitivity of the detector reaches the quantum noise limit. The mixing efficiency is vital for the intensity of the intermediate frequency signal. Therefore, we assume that the local oscillator light has an ideal Gaussian distribution and calculate the influence of different amplitude distributions of the signal light on the mixing efficiency. By designing a set of beam-shaping mirrors, the amplitude distribution of the signal light is changed to a Gaussian distribution, coherent mixing of Gaussian light and Gaussian light is achieved. Experiment results show that the coherent detection mixer with shaping elements can achieve the similar amplitude distribution of the signal light and the local oscillator light, and the mixing efficiency is improved by 17 percentage points compared with the uniformly distributed signal light.