Abstract: A special high pressure humidifying test-bed was established to research the design of humidifying system for the fresh-keeping transportation. The regulation characteristics of high pressure humidifying system were studied. The effects of airflow velocity, ratio of open area and length of the return air path on the characteristics of humidify efficiency and humidity distribution were analyzed. Some relationships were found out. According the calculation formula of corrugated carton compression strength, the reliability of the design of pressure room and perforated baffle was demonstrated. The relation between humidify efficiency and airflow velocity, ratio of open area and length of the return air path humidity was nonlinear. At the same airflow velocity and ratio of open area, the longer the return air path, the minor humidity uniformity was. The optimum parameters of the high pressure humidifying test-bed were determined by experiment, which required the ratio of open area was 7.56%, the airflow velocity was 8m/s, the return air path length was 1.5m. The research achievements were instructive to the optimization design of the humidify system for fresh-keeping transportation.