In order to prevent the instability of the tunnel face due to improper support pressure control during the construction of the shield water transmission tunnel, a three-dimensional numerical model is established by FLAC^3D software based on the follow-up optimization project of the connection of the Hanjiang-Rongjiang-Lianjiang water system. Aiming at the four typical strata traversed by the shield water transmission tunnel, the stability of the tunnel face during shield construction under different support pressure conditions is simulated and analyzed, the development law of the tunnel face and its front plastic zone under various working conditions is studied, and the lower limit of the ultimate support pressure of the tunnel surface in various layers is further determined. The results show that: 1) The sensitivity of four strata, namely silty soil, clay, argillaceous coarse sand and argillaceous siltstone, decreases in turn; 2) When the support pressure ratio gradually decreases, the plastic zone develops towards the tunnel surface, and when the tunnel surface is unstable, a logarithmic spiral plastic zone will appear in front of it; 3) For the three types of silty soil, clay and argillaceous coarse sand, the ultimate support pressure ratio and recommended values are 0.69 (recommended value is 0.90), 0.57 (recommended value is 0.80) and 0.41 (recommended value is 0.65), respectively. For argillaceous siltstone formations, the tunnel face has self-stabilizing ability, and the recommended support pressure ratio is 0.6. The results can provide reference for the construction of shield construction in similar projects.