Abstract: The automatic control device using single-chip processor as a key control component for rotary cylinder crop seed dryer was developed in order to increase the safety of seeds and control precision. This paper briefs the construction and principle of seed dryer, discusses the function of automatic control device in the process of seed drying. The control circuit was designed and the program for automatic control device was developed. The experimental results show that the control device has an advantage of even drying, high speed and efficiency, ensuring seed vitallity. The construction is reasonable and can meet the requirements of seed safety and control precision in the process of seed drying.