Emerging technologies usually could change the structure of a industry.The characteristics of emerging technologies are illustrated and "marginal utility of technological innovations" is put forward.Based on the theory of S curve of technology by Foster,a new concept "the filter-line for an emerging technology" is proposed.The concept is used to differentiate emerging technologies from non-emerging techologies.The "filter-line" is a usable tool for an enterprise to select emerging technologies when confronting with a number of new technologies.