Based on the data of a long term field experiment of dryland crops, this article points out that yield fluctuation, expressed in fluctuating coefficient, is a regular phenomenon in dryland farming.High-yield field has a similar fluctuation as compared with low-yield field but their mechanisms are different.Desiccation happened in the entire soil profile is a direct cause of the yield fluctuation for high-yield field, but low water absorbing capacity of the crop for low-yielding field.