Abstract: A new analytical method, was presented to calculate the diameter and height of spray dryer in order to improve the tedious process of diagram integral method which must read the nomogram repeatedly and overcome the difficulty of diagram integral method which could not be programmed to calculate. The analytical equations of horizontal flying distance and time and the decelerating vertical falling distance and time of the fog droplet were derived by analytical method in the transition region(most of the fog droplet movement in the spray dryers was in this region). The analytical equations for calculating the fog droplet movement distance in the laminar region and the turbulent region were also derived by analytical method. The diameter and height of spray dryer could be calculated by combining these analytical equations with the analytical equations from references for calculating the fog droplet movement time in the laminar region and the turbulent region. The calculation example indicated that the analytical method was more precise and convenient than the diagram integral method, and very convenient to apply to artificiality or computer calculation, and its calculation precision could meet the requirement of engineering design.