Based on the theory of soil plasticity,a model of the passive earth pressure from trapezoodal-cross-section finite soil body was built. The proportional relation and the effect of the model parameter on relationship between the passive earth pressure from trapezoodal-cross-section finite soil body and half-space passive earth pressure were researched. A reinforcing deep excavation was designed in Rankine theory and the model introduced in the paper.The effect of the passive earth pressure from trapezoodal-cross-section finite soil body on the stability of deep excavation and municipal slop was revealed,which verified the calculation results by the model.