Abstract: Ultra-deep oil and gas will be the main field of China's future energy development. There are many unique drilling problems in myriametre ultra-deep wells. Among them, in order to reserve enough space for the lower part of ultra-deep, the upper part of the ultra-deep well needs to be drilled with super large hole size. However, the existing oilfield drilling design methods do not consider the influence of hole size on the stability of borehole wall, which leads to inappropriate drilling fluid density and the serious collapse of large upper hole. In view of this, the applicability of existing borehole stability evaluation methods in large boreholes are discussed by combining conventional analytical model of borehole stability and numerical model of borehole stability with different borehole sizes, and the law of influence of borehole size on borehole stability under homogeneous and fractured formation conditions is clarified. It is concluded that the influence of hole size on rock stability is small under homogeneous conditions, triggering little increment of collapse pressure. The existing analytical method of wellbore stability has good applicability. However, in homogeneous situation, borehole size can increase the area of wellbore instability, causing serious collapsing. When the formation is rich with fractures, it is easy to cut more weak structural planes in the large-size hole, which leads to the stress fluctuation at wall of borehole, decrease of rock stability and the increase of the failure area around the hole and failure area has irregularity. Besides, the collapse pressure increases dramatically, aggravating the problem of wellbore instability. With increasing development degree of fracture, influence of borehole size on wellbore stability becomes stronger and wellbore stability weakens. The existing analytical method of wellbore stability is not able to consider the disturbance of borehole size on wellbore stress, thus it has poor applicability in fractured formation. On this basis, the size effect of wellbore stability is proved through different drilling cases. In particular, for wellbore stability evaluation before drilling on the basis of nearby well, in formation with abundant fractures and borehole size has large change, the wellbore size difference between nearby well and upcoming drilling well has to be considered. The research results have deepened the theoretical system of ultra-deep wellbore stability, and offers theoretical support for optimization design of ultra-deep drilling in China.