Abstract：This paper focuses on the applications of extension theory in geological engineering. The characteristics of geological engineering are first discussed, and it is shown that the effect of different grades on the correlation degree is equivalent but interval lengths may be different; and correlation degree is linearly related with effect factor. With a special and simple geological engineering example, the extension matter-element theory is applied and then expanded, and related curves of all grades between correlation degrees and grades (effect factor values) are plotted. The problems are analyzed based on differences between curves and actual situations. A new correlation function is then proposed to improve the differences between the original function and actual situations. Finally, the improved function is shown in agreement with actual situations by analyzing its curves in geological engineering.