Abstract: This article analyses different reasons of causing air resistance phenomenon. An conclusion has been made by practice and analysis of plenty hy draulic calculation witi drawings and tables. That is to say, it is easy to do and has elfective result to overcome air resistance by reforming the crane. Thus, it has great directive sense for the practical work in analysing macroscopically whether air resistance has been appeared during oil unloading by adopting the concept of introduced critical tempreture