Abstract: In order to avoid caprock integrity failure caused by unreasonable water injection pressure during water injection in offshore oil fields, it is necessary to conduct an uncertainty analysis on extreme injection pressure and the controlling influences of sensitive factors on extreme injection pressure. In this paper, a water injector in the PL19-3 Oilfield was taken as a research object and a caprock model was established by using FLAC3D software. Based on this model, the extreme water injection pressure (i.e. the water injection pressure when the caprock is initially broken) was calculated. Then, the effect of sensitive parameters (e.g. permeability, cohesive force and internal friction angle) on extreme injection pressure was analyzed. Furthermore, the uncertainty of extreme water injection pressure was studied by means of Monte Carlo simulation (MCS). It is shown that cap rocks begin to break when the extreme injection pressure is reached. As the permeability increases, extreme injection pressure varies, first in the trend of increasing, decreasing then increasing slowly. The extreme injection pressure increases linearly as the cohesive force increases, but in the form of quadratic function with the increasing of the internal friction angle. A deterministic result of extreme injection pressure is only a part of the uncertainty analysis. Through uncertainty analysis, the range of extreme injection pressure with higher probability of caprock failure can be revealed and extreme injection pressure can be evaluated comprehensively so as to provide the theoretical guidance in the secure management of water injection.