Water seepage damage is prevalent at the Lei Gutai grotto of Longmen grotto. To better understand the mechanism of water seepage, 149 discontinuities at this area were investigated. Taking these discontinuities as sample parameters, a 3D rock discontinuity network model was built, which aids the study of rock mass innerbody connectivity. Using this discontinuity network it is found that the major connective path at Lei Gutai consisted of three group discontinuities:one stratum (trending NNW) and two high dip discontinuities (trending) and that their degrees of connectivity are up to 87.1% and 83.6% at middle and south parts of Lei Gutai, respectively. Combining these results with the 3D model, inside and outside field discontinuities are investigated, and finally developed a mechanism for water seepage formation at the Lei Gutai grotto. The north grotto has two discontinuities, J12 (trending SE) and J4 (NNW), that pass through the No.2 reserving rainfall platform, which allows surface runoff to infiltrate into the upper rock mass of the grotto. Next, the internal discontinuities, trending south east (SE) and south west (SW), stretch upward and intersect with J12 and J4. Consequently, surface runoff along those internal discontinuities seeps into the north and middle grottos. J12 and J35 control the entrance of surface runoff into it, and many internal discontinuities, trending SW and SE, connect with its seepage channel. When it rains, surface water permeates into the top of the grotto through J12 and J35, and then flows through internal discontinuities onto the internal surface of the grotto. For the south grotto, considering mountain is deficiency, rainfall cannot infiltrate into the grotto along with stratum, and water arriving into internal surface is from the interval of manual brickwork and mountain, and discontinuities such as J11 (trend SE), J31, J30 (trend SW). Once rainfall through these cracks infiltrates unto the grotto above, internal connective discontinuities of south grotto (most trend SW) will guide water seeping to its surface.