Using the ideal segmentation of the gas and liquid two-phase flow within the Y-jet nozzle, a multistage, one-dimensional separate-phase model is presented in this article to describe the mass, momentum, and energy conservation of gas and liquid flows, with interactions of the two phases at diffrent flow paths of the nozzle. Combined with the general two-stage, uniform atomization model outside the nozzle, a complete model sVstem for the single uniform diameter prediction is formed. Furthermore, the random characteristics of the liquid sheet primary breakup are described with the Monte Carlo method, and an improved model is proposed here to predict the droplet size distribution for the Y-jet nozzle. To vertify the model system, a laser-diffraction size-distribution analyzer was applied, and several different Y-jet nozzles were tested. Very good agreement between the experimental results and the model predictions are obtained and presented in this article.