Coating has widely been a commonly-used way to prevent corrosion failure of machinery parts. Coating of agricultural machinery parts is also conducive to improving the working reliability and service life of machinery. Drying is thus the key link of parts coating. However, improper drying can cause the reduction of film stress, film cracks, shrinkage, and pinholes. In this study, a side air supply drying room was designed suitable for the drying of agricultural machinery parts. The circulation of hot air was used to exchange the heat with the workpiece, further to reduce the moisture content of the coating, and finally to realize the curing of the coating. Computational fluid dynamics software was selected to numerically simulate the velocity and temperature field of the drying room for agricultural machinery parts. The working parameters of the drying room were also optimized to clarify the influence on the velocity and temperature field. As such, the working performance of the drying room was improved to clarify the influence of air supply mode, the number of air supply outlets, and air supply angle on velocity and temperature distribution. The results show that the unilateral air supply performed better than that of the double, indicating better gas circulation in the drying room. The uniformity of speed and temperature was better, particularly with the higher speed of return air and excellent gas circulation, when the number of air inlets was 8 rather than 6 and 10. Furthermore, the uniformity of speed and temperature was better, when the air inlet angle was 15° rather than 0° and 30°, where the return airspeed and temperature were higher. Therefore, an optimal parameter combination of drying room was achieved, where the unilateral side air supply, 8 air supply outlets, and air supply angle of 15°, indicating excellent gas circulation, as well as the better uniformity of speed and temperature. The measured values of each index were in good agreement with the theoretical optimization, where the temperature error was less than 1.68%, and the wind speed error was less than 33%, indicating reliable and effective simulation. At the same time, the overall coefficient of temperature inhomogeneity was less than 6%, where the designed drying room worked stably and reliably for agricultural machinery parts, fully meeting the design requirements. This finding can provide a strong reference for the design of the subsequent drying rooms.