In order to meeting the demand of measuring the water mist droplet size with high accuracy and low cost,the machine vision measuring technique for the droplet size was studied in depth. The water mist droplets were sampled in self-building high pressure spraying system and droplets collecting device,the photos of the droplets sample were taken by the microscope and its CCD camera,the droplets sample photos were processed and analysed using the image processing software. The total number of the measured and counted droplets was 5966. The droplet size spectral,cumulative distribution and the droplet mean and indicated diameter were obtained. The machine vision measuring results were compared with the PDPA measuring results. Results indicate that the minimum droplet which the machine vision technique can be used to measure is 4.39 μm,and the machine vision results are close to the PDPA measuring results.The relative errors of the water mist droplet mean and indicated diameter are less than 5%,and that of the droplet size dispersion coefficient is 0.27%.