Partial leakage occurs occasionally in the undersea shield tunnels or high groundwater level shield tunnels after long-term use. Seepage effects caused by partial leakage are rarely considered in the existing theory of loosening earth pressure. Therefore, on the basis of the previous research, the seepage field is analyzed in the case of the leakage of the segment joint. This study proposes an analytical model to evaluate the loosening earth pressure based on Terzaghi’s assumption, considering the characteristics of loosening zone shape of deeply-buried shield tunnels and the seepage effects of the joint leaking. The analytical model is verified via a parametric analysis and using a FLAC numerical model. The result show that when the distance between the leaking joint and tunnel vault is high, the loosening earth pressure acting on the tunnel is high; higher surface water head leads to stronger weakening effects of the soil arch. The loosening earth pressure increases with the increasing seepage quantity of the leaking joint; the higher the permeability of the strata, the smaller the loosening earth pressure at the top of the tunnel; the separate calculation method strengthens the arching effect compared to the combining calculation of water and soil pressure; the theoretical results are consistent well with the numerical results.