The lateral pressure coefficient and formation hardness affect the force of shield tunnel segments. To clarify their influencing patterns, a model test platform that can realize pressure loading at formation pressure zones is developed, and a model test of large-scale indoor segment force is conducted. Additionally, numerical simulation of segment force is conducted simultaneously to examine the effects of lateral earth pressure and formation modulus on soil pressure, internal force, and deformation of segment structure. Findings are as follows: (1) The influence of the lateral pressure coefficient on the earth pressure at the top of segment is significantly greater than that of other parts. With an increase of the lateral pressure coefficient, the absolute bending moment and axial force at the top of segment decrease first and then increase, and the values change from positive to negative. (2) In the vertical loading stage, the top of segment undergoes a state of inner tension and outer compression, and the absolute strain gradually increases. With the intervention of horizontal load, the strain of the segment shows an inflection point, and the strain first decreases to zero and then increases in reverse, showing a state of inner compression and outer tension. (3) The lateral pressure coefficient has no significant effect on the maximum displacement of the segment, but has a great influence on the maximum stress. The maximum stress of the segment at the lateral pressure coefficient of 0.8 is about 1.8 times that at the lateral pressure coefficient of 0.3. (4) The maximum stress and displacement of the segment in the uneven soft and hard strata are between those in hard and soft strata, closing to that in soft strata, and the maximum displacement of the segment in the hard stratum is the smallest.