The finite element numerical simulation method is used to study the layout modes of the cavern enlargement with and without vertical water curtain system by taking a largescale underground watersealed oil storage cavern as an example. The study results show that: (1) When the distance between two caverns constructed at different time is small, the vertical water curtain system can effectively reduce the influence of the proposed cavern on the watersealed reliability of the completed one; with the increase of the distance between the two caverns, the influence of the vertical water curtain gradually weakens. (2) The influence of the axial direction of the proposed cavern on the watersealed reliability is greatly affected by the terrain; moreover, in this case the vertical water curtain system has less influence on the watersealed reliability. (3) With the increase of the depth of the proposed cavern, the watersealed reliability of the proposed cavern gradually increases, while the watersealed reliability of the completed cavern decreases gradually; the vertical water curtain system between the two stages of the caverns can effectively reduce the effect of the depth on the watersealed reliability.