Abstract: The 17¹/₂"（Ø444.5mm） large size boreholes of Ultra-deep well in Shunbei block was a long section, there were not only the problems of speed raising and deviation control, but also the frequent failure of box end of stabilizer. Based on the analysis of pendulum bottom hole assembly（BHA） dynamics, this paper discussed the failure mechanism of box end of stabilizer in large size boreholes. Firstly, based on the dynamic finite element equation of drill string, the complex dynamic characteristics at the stabilizer of BHA in large-size borehole were studied, and the dynamic bending moment and vortex characteristics near the box end of stabilizer were determined. Secondly, a three-dimensional finite element model of the thread joint at the box end of the stabilizer is established, and the stress distribution characteristics of the thread joint under complex load were analyzed by using the explicit dynamic finite element method. Finally, the dynamic stress near the variable cross section position of the BHA stabilizer's box end was deeply analyzed by taken the failure of the box end of stabilizer in Shunbei XX well as an example. The results show that the axial and torsional vibration level of the box end of the stabilizer were low, but there was a strong high frequency vortex, and a high frequency additional dynamic bending moment（0~453.0 kN·m） aroused near the box end of the stabilizer. This bending moment produced a maximum stress of 799.1 MPa on the thread of the box, although the value was less than the stress (973.1 MPa) on the thread of the drill collar pin engaged with it, the amplitude of variation was large (465.0 MPa) and the frequency was high. This high frequency variation stress was one of the key reasons to cause the failure of the box end of the stabilizer. Hence, it is suggested to control the vortex and minimize the dynamic bending effect near the stabilizer when designing large-scale borehole BHA.