A theoretical model for predicting the droplet size distribution for electrostatic spray was described based on the information entropy method. The model was numerically calculated using the modified Newton-Raphson method. The present model can evaluate the droplet size distribution for a cone-jet mode electrostatic spray. The predicted results of present model show that the droplet size distribution exhibits a narrower and monodisperse behavior and the ratio between the largest diameter and the minimum diameter of droplets is between 2 and 3. The comparisons show that the model is in better agreement with experimental data except for the case under smaller flow rate. The research results of present work are helpful to optimization and control for the electrostatic spray process.