In order to ensure the highest content of effective components and reduce the energy consumption in drying process, a constant temperature control system with double closed-loop PID control is designed, which is composed of heating system, heat storage system, drying system and monitoring system. The target value and measured value of inner loop PID are respectively fed into terminal No.2 and terminal No.4 of the frequency converter. After calculation, the operation amount acts on the air energy heat pump to realize water temperature regulation of the water tank. The temperature setting value and measurement value in the outer loop PID room are respectively put into the analog channel of a special adapter FX3U-3A. After PLC calculation, the amount of the output controls the on-off switch of SSR, which drives the fan in the drying room to operate, and realizes the constant temperature drying demand.