Abstract: The microscopic characteristics of water-in-oil emulsion in a flow system play a key role in investigating its rheological property, wax precipitation characteristic and formation performance of hydrates. In this paper, based on a high-pressure hydrate slurry flow loop, the influence of water content, flow shear rate, temperature and pressure on the microscopic characteristics of water-in-oil emulsion droplets before the formation of hydrate were analyzed under different experimental conditions by using the online high-power particle size analyzer. It is indicated that water-in-oil emulsion droplets are smaller, more stable and dispersed more uniformly in the system with lower water content and pressure and higher flow shear rate and temperature. Then, the effects of temperature, pressure, flow rate, water content, density, viscosity, surface tension and thermal conductivity on the size and distribution of droplets were analyzed comprehensively, and a formula was established for predicting the average size and proportional distribution relationship of dispersed droplets in water-in-oil emulsion in three scales. And finally, prediction results were compared with the experimental data to verify the correctness of the formula. It is shown that the predication precision is within ±5%. It is expected to provide the technical support and important basis for the further dynamics mechanism analysis and model prediction on the formation of hydrate in flow systems based on hydrate shell model.