In view of the present commonly used fuzzy theory integrated well-to-well interporosity flow evaluation method，the ratings appear contradictory of each single index，and the accuracy of the discriminant method of the fuzzy theory is greatly affected.This paper tries to apply extension theory to the researches on cross-flow between the wells assessment.According to the characteristics and influences of water-flood in fracture reservoir，and on the basis of matter-element theory，the extension set theory and dependent function calculation，the characteristics of cross-flow between the wells are analyzed by means of qualitative and quantitative method.By determining cross-flow field matter，classical field matter，elements and segment field matter，weight coefficient and correlation function，which applied extension decision method，cluster analysis and gravity method，the Extenics theory fully considers the various factors that are interrelated and interdependent，and it can turn the evaluation results incompatibility of interaction characteristics of the single index into compatibility，and then it is distinguished by extension theory method，it is proved to be feasible by a practical application in Yangjiaba reservoir with 86% accuracy，and it is promising in application.