In a layered lithosphere slipping system, there are many pores filled of fluid with a certain pressure. Suppose the floating force caused by the pore fluid pressure in the slipping plane is Ff, the loading weight of bedding slipping block is ST , then a new index of R ,the ratio of Ff to ST , can be given, that is very important to describe the slipping motion of the layered lithosphere being easy or difficult. Actually, shear strain of the lithosphere can outstandingly change its value. In this paper the author try to inference a dynamic equation describing the relationship between R and the shear angle (α) as the follow formula: R(α)=R0P(α)P0cosα (1) R(α)=R0cosα,\[P(α)=P0，constant pressure\] (2) According above formula(1), R(α) is positive correlation with P(α), and also is positive correlation with shear angle α. It is difficult to give the formula of describing the relationship between R(α) and α, but the pore fluid keeping its pressure to be a constant is common case in the shearing process, that is P(α)= P0. P0 replaces P(α) of above formula(1), the above formula(2) can be gotten, and then R(α)—α diagram of constant pressure shearing process can be drawn, in which the law can be discovered as follow: in a constant pressure shear strain process, when R0>0, R(α) increase with α; as shear angle α increasing at a certain value, R(α) exceed 1 rapidly; and the move R0 is, the less the α is for R(α)>1; otherwise, the more the α is. R(α)>1 means the bedding slipping block is totally supported by pore fluid pressure. This law reveals a possible outbreaking mechanism of subduction—thrust nappe or detachment structure or landslide in lithosphere.