Steel tubular pile underpinning technology is an effective method to develop underground space in retrofitting of existing buildings. The bottom of steel tubular pile inserted into concrete pile while top connected with underpinning cap. The stability of the pile under compression load decreased after the soil around the pile dug

but the stability coefficient and length factor of steel tubular pile can not be obtained by currently code. According to the 1:2 model test and nonlinear buckling analysis respectively

the stress distribution and the ultimate bearing capacity of the pile is derived. The effective length factor for stability calculation is 0.616 gotten by test study; and the numerical result is 0.683. So the pile is restricted with base edge fixed and top edge articulated. More safety stability of the pile can be obtained by buckling numerical analysis which can provide reference for pile design.