Abstract: Keller Uniformity is recommended by USDA-SCS to design micro-irrigation system. In order to discuss whether the method is correct or not, Keller Uniformity and the design method of micro-irrigation system were discussed in detail according to the definitions of Keller Uniformity and hydraulics principle of multiple-outlet pipeline. The comparative analysis of the two definitions of Keller Uniformity indicates that there is substantive and completely difference between the two definitions, which cannot be confused. The formula of Keller Uniformity using field measured data (definition 1) is only used as an after-evaluation indicator of micro-irrigation system. The micro-irrigation system design using Keller Uniformity was based on manufacture deviation index (definition 2), and the later definition is only the outflow characteristic of the minimal working pressure point. As a design index, it is not reasonable. In this method calculated evenness(EU) was replaced by recommended Uniformity (EUT), which leads to the flow expectation calculation formula of emitter under the smallest-pressure lack of scientific basis, while the calculation formula of allowable pressure head differences does not match with the hydraulics law of multiple-outlet pipeline. The cause for the mistakes was that the hydraulics knowledge of multiple-outlet pipeline was incomplete at that time, while the practice of micro-irrigation urgently needed a design method. So up today this design method should be re-examined.