The Chang 8 reservoir in Jiyuan Oilfield exist a large number of under-injected wells, most of which are with high under-injection degrees and complicated influence factors, this result in low formation energy and fast decline of oil productions. In order to solve this problem, based on geological and development factors, systematical analyzes on the reservoir permeability, reservoir microscopic characteristics, sensitivity, wettability, injection water quality and water compatibility are conducted to illuminate the influences on the under-injection wells; then, the PCA (principal component analysis) and the maximum membership degree methods are combined to establish the main controlling factors of the under-injection discriminant method, with 133 groups of data from Jiyuan Oilfield taken as samples, the influence weight of 24 key characteristic parameters are determined. It is found that the reservoir physical property, injection water quality and water compatibility are the most significant influential factors to lead the under-injection problems of Jiyuan Oilfield. The application shows that this method can identify the main control factors of under-injection quantitatively and accurately, and it can provide a reliable guidance for the subsequent treatments of the under-injected wells.