According to the characteristics of longan, a forced-air pre-cooling device of longan for test was designed. Based on 7/8 time of pre-cooling, the relationship between air velocity and pressure drop or voltage frequency was obtained in test, and furthermore, the relationship between air velocity and energy was studied. The results show that the pre-cooling fan works more slowly, but costs a long time with a lower level of energy cost. In reality, a less time consuming method was needed. The method is controlling the increase of the air velocity with the decreased temperature of longan, the relationship between them was described, and the test proves the conclusion is reasonable.