The droplet size is an important indicator of performance evaluation of plant protection spray and work quality machinery. It is the key technical parameters to achieve pesticides zero-growth and precision spraying. The study of best biological particle size in pesticide use and the study of immersion models and fog droplets evaporation deposition models are related to the size of the fog droplet size. This paper summarizes the various fog droplets test techniques. The fog droplet diameter of Φ0.3 mm, Φ0.7 mm, Φ1.0 mm, Φ1.3 mm, and Φ1.6 mm are measured and evaluated. Analysis of the sources of uncertainty during the measurement process were performed. The results show that when the inclusion probability is 95%, the minimum expanded uncertainty of the measurement results is 6.63%, the maximum expanded uncertainty is 9.18%, and the average expanded uncertainty is 7.38%. The laser particle size analyzer is accurate and reliable in measuring the particle size of the droplets, which can meet the needs of the droplet particle size testing of plant protection machinery.