In order to study overriding and cutting off coupling effect in process of joint shear, the joint three-dimensional models which have asperities of different combinations of height (h) and angle (?) are made for, conducting model shear tests. The test results confirm this coupling effect. The joint shear movement characteristics are analyzed; and a ratio of cutting off area Sj to shear area S, i.e. K, is proposed to present quantitative characteristics for the degree of coupling effect. In different dominant effect, the K changes between 0-1. Meanwhile, a shear strength formula which is involved in the coupling effect of overriding and cutting off is established to help valuating effect rule of joint shear strength. The results show not only that shear strength is largely determined by overriding friction when the K is low for hard rock with large compressive strength and for slightly undulated joint surface, but also that it is largely determined by strength of the rock material when the K is high for soft rock with small compressive strength and for roughly undulated joint surface.