Due to setting of the bearing push-extend reamed of the Push-extend Multi-under-reamed Pile, the stress of soil that the upper and bottom of the bearing push-extend reamed changes at the state of compression. So there are unreasonable parts in the present formula of ultimate bearing capacity of the single pile. The ultimate bearing capacity of the soil around the Push-extend Multi-under-reamed Pile is decided, in the article, according to the failure principle of soil and the theory of sliding line, at the same time, to combine the theory of plasticity potential energy with the principle of imaginary. So the calculating model of calculating end resistance and side friction resistance of pile is corrected and perfected and the up-to-date formula of ultimate bearing capacity of the single pile is put forward at the compression.