Abstract: Abstract: Consumption of petroleum resource has risen sharply in recent years with the increase in fuel vehicles and heavy machinery in mechanized intensive agriculture. A large amount of emitted pollution has posed a serious threat to the atmospheric environment, even to body health in human survival. Water-in-oil (W/O) emulsified diesel fuel has received widespread attention for its ability to simultaneously remove nitrogen oxides and particulate matter. The micro-explosive characteristics of W/O fuel depend strongly on the diameter of the water core. But a systematic solution is still lacking in the preparation of W/O emulsified fuel for the target diameter of the water core at the constant component content. In this study, an image recognition system was first developed suitable for the microstructure of the water core in the W/O emulsified fuel using the Chan-Vese (CV) model. A log-normal function was proposed to fit the diameter distribution of the water core in the W/O emulsified fuel, thereby obtaining a new combination of characteristic parameters. An orthogonal test was selected to explore the influence of emulsification parameters on the diameter of the water core, including the emulsification time, emulsification power, and frequency parameters. The optimal parameters were achieved to fabricate the W/O emulsion with the target diameter of the water core. The results show that the CV model better identified the discrete boundary of the water core with smaller error, compared with manual recognition. The characteristics parameters from the lognormal function fitting can describe more accurately the diameter distribution of the water core in the W/O emulsified fuel, compared with the Sauter Mean Diameter (SMD). The maximum diameter of the water core and the standard deviation of diameter distribution gradually decreased, while the ultrasonic frequency imposed a relatively weak influence on the W/O emulsified fuel, as the emulsification time and power increased. When the diameter of the water core and the distribution reached a critical value, there was much more effect of emulsification power on the diameter of water core and the distribution, compared with the emulsification time with the same energy. In addition, there was no change in the maximum diameter of the water core. The ratios of three preparation factors (ultrasonic frequency, emulsification power, and emulsification time) to the maximum fitting diameter were 0.744, 2.880, and 1.038, respectively. The standard deviations of distribution were 0.028, 0.120, and 0.034, respectively. The priority of three parameters was ranked in order: emulsification power, emulsification time, ultrasonic frequency.