Problem of "pit in pit" (PIP)is a universal problem which hasn't been researched deeply. It is the realistic meaning for the PIP's equivalent conventional design. According to the rules of equivalent outer protective structure's maximum deformation, the feasibility of the equivalent method was studied. Through analyzing three important parameters:pit toe coefficient(
χ), the area ratio of inside and outside(
α), the depth ratio of inside and outside(
β), a formulas is derived to calculate PIP's equivalent design depth.