Artificial ground freezing is a common green construction method for strengthening the strata of soft clay in coastal areas. Due to the large-scale and complexity of underground excavation, the artificial ground freezing is faced with more complicated seepage situations. By extending the three-dimensional rigid ice model of the segregation frost heave theory, a thermal-hydraulic-mechanical coupling frost heave and thawing settlement model considering water migration was established to simulate artificial freezing under complex seepage environments. Based on the model, simulation was carried out by using COMSOL finite element, and results were compared with the experimental data of the indoor model of the same size, so as to verify the applicability of the model in the freezing of combined stratum (soft clay, silt fine sand) in complex seepage environment. Simulation results show that under combined formation conditions, seepage reduced the thickness of the freezing curtain when freezing was stable; when the speed was greater than 1.2 m/d, the freezing curtain could not reach the design thickness; the frost heave force on the side of the freezing curtain near the seepage boundary increased slowly and the maximum value was low; seepage caused uneven settlement on the ground, and the settlement in the downstream area was greater. Finally, the verified model was used to simulate and predict the construction of the freezing method under the action of combined stratum seepage, and the grading standard of the tunnel safety freezing method combining the combined stratum seepage factors was proposed to provide the basis for the construction safety and early warning of the freezing method in more complicated coastal seepage environments.