For study the influence caused by geobelt and the mechanism of the strength increment in the reinforced foundation, the author analyzed stress state in each zone with reinforced and unreinforced, combined with the Mohr-Coulomb yield criterion, found the stress variation relationship between and in both foundations, which are found by the stress analysis method and geometry analysis method under the extreme state condition. The extreme normal stress incremental of vertical direction was solved in reinforced foundation. The essence of strength increment in reinforced foundation is revealed. The reliability of theoretical analysis was proved by numerical simulation. The conclusions show that increases in the active extreme equilibrium zone I and the passive extreme equilibrium zone Ⅲ, increases upper geobelt and decreases under geobelt in the transition zone ; the normal stress of each zone have changed correspondingly, the incremental variation of in I zone and the in Ⅲ zone has pseudo-cohesion.