Clouds cover two-thirds of the Earth's surface and have an important impact on the global radiation balance, global climate change, hydrological cycle, and artificial weather modification. Meanwhile, the cloud in the atmosphere remains one of the biggest uncertainties in weather and climate changes. As cloud microphysical parameters, number concentration, median volume diameter and liquid water content are important parameters to investigate cloud microphysical processes and weather prediction. In the current observation technology of cloud microphysical parameters, remote sensing method exploits the power spectrum data of satellites and radars to invert cloud microphysical parameters. However, in the progress of data inversion, properties of cloud droplets need to be assumed. Therefore, realistic droplet spectrum and cloud microphysical parameters cannot be obtained, and their measurement accuracy needs to be further verified. Airborne instrument requires strict airspace application, and its observation time, spatial continuity and sampling frequency are limited. The measurement method of existing foreign droplet spectrometer based on light scattering will destroy the original cloud droplets field distribution. In view of the above bottleneck problems, an orographic cloud observation method based on digital holographic theory is proposed. This observation method combines the active wind direction follow-up system and the nanosecond pulsed laser modulation technique based on complex programmable logic device, and utilizes global digital image fusion and local tenengrad variance algorithm. The digital holographic experimental system based on this method uses a nanosecond pulsed laser light as the light source. It can eliminate the multiple ghosting phenomenon of high-speed moving particles and obtain accurate holographic images during the recording process of holograms. In the reproduction process of holograms, global digital image fusion and local tenengrad variance algorithm can determine the focus position of particles in the measurement space to obtain more accurate three-dimensional coordinate and size of particles. For traditional fog monitor based on the light scattering theory, its sampling method is inspiratory, which causes the loss of particles during the sampling process of particles. However, the sampling method of the digital holographic experimental system is open. The active wind direction follow-up system can avoid particles loss to obtain the more realistic droplet spectrum. In the Liupan Mountain Orographic Cloud Field Science Experimental Base, long-term continuous observation is conducted to obtain cloud microphysical parameters. These observation data are compared and analyzed with the observation data of a light scattering-based fog monitor and forward scatter visibility instrument. In three comparative experiments, for particles of 2~4 μm, the measurement results of the fog monitor are 61.54%, 30.24% and 18.39% of the digital holographic system, respectively. For particles of 7~50 μm, the measurement results of the fog monitor are 26.90%, 16.79% and 28.57% of digital holographic system, respectively. The above results show that the digital holographic method measures more droplets in the interval of 2~4 μm and 7~50 μm. In recent years, many researchers have found that FM120 has particle loss during measurement, which is consistent with the findings of this paper. This method can lay data support for improving the theoretical understanding of the physical process of cloud precipitation and the development of parametric schemes. It can also provide important technical support for research in the fields of weather, climate, artificial weather modification and atmospheric chemistry.