Abstract: The results of the triaxial compression tests with solid confining pressure media and the theoretical analyses show that the "environmental stiffness",including the stiffness of both axial and confining pressure systems,is an important factor which influences the stress drop. Under the condition of triaxial compression tests the axial stress drop is a hyperbolical function of the stiffness of confining pressure system,and the former decreases with increasing the later. Thus,the problem,that the experimental stress drop is much greater than that estimated from an earthquake,may be explained.The expression of shear stress drop of hypocenter in the crust is Δτ=2λ((S_s—S_k)+(μ_s—μ_kσ_n.0)where λ is the environmental factor which depends mainly upon the environmental stiffness and the orientation of stress field. The values of the environmental factor may be 0-0.9 for the conditions without obvious change in the magnitude of normal pressure and the orientation of principal stresses.The environmental stiffness not only influences the magnitude of stress drop,but also determines whether the sudden stress drop will occur or not. Fully taking account of the effect of environmental stiffness and other factors such as temperature,confining pressure,pore pressure,orientation of stress field,fault materials,etc.,it will finally be possible to recognize the total law of stress drop in hypocenter.It is also indicated in this paper that the direct shear tests and triaxial compression tests with gas or liquid confining media can only model the pressure condition of hypocenter,but cannot reflect completely the "environmental stiffness" which can be controlled to a large extent with solid confining media as presented.