In the process of laser propagation in atmosphere, its coherence characteristics are reduced by turbulence, which further affects the efficiency and performance of coherent detection process. In this paper, atmospheric coherence time is defined to describe the fluctuation speed of laser field after atmospheric transmission. The relative size of atmospheric coherence time and coherence process duration time determines the performance of coherent detection process. According to the infinitely long phase screen technology, we simulate laser atmospheric transmission, and systematically study the influences of atmospheric parameters, transceiver parameters and wavelength on atmospheric coherence time. It is found that the atmospheric coherence time is positively correlated with wavelength, receiving aperture and atmospheric coherence length, and inversely proportional to wind speed, which shows that the atmospheric coherence time can be effectively improved by improving the optical design and changing the laser band, thus effectively reducing the disturbance caused by turbulence and improving the stability of the received light field. The atmospheric coherence time defined in this paper is an important parameter to measure the influence of turbulence on coherent detection. This study can provide a powerful reference for evaluating the influence of atmosphere on coherent detection process.