Existing fatigue-based topology optimization methods are primarily developed for linear elastic materials, while studies on bi-modulus materials, such as concrete and fiber-reinforced composites, remain limited. However, the inherent tension–compression asymmetry of bi-modulus materials can significantly alter stress distributions and fatigue damage evolution. Neglecting this characteristic may lead to inaccurate fatigue life predictions and result in non-conservative or even failed fatigue-optimized designs. Therefore, this paper proposes a systematic fatigue-based topology optimization framework for bi-modulus structures subjected to proportional cyclic loading. For geometric representation, the explicit Moving Morphable Void (MMV) method is adopted to ensure clear structural boundaries. Meanwhile, a computational framework based on the consistent tangent stiffness matrix is integrated to accurately capture the nonlinear mechanical response induced by the bi-modulus constitutive behavior and to achieve quadratic convergence. High-cycle fatigue damage is evaluated using the Dowling–Basquin model combined with the Palmgren–Miner rule, and a P-norm aggregation function is employed to construct a differentiable global objective function. In addition, sensitivity analysis is performed using the adjoint method. Numerical examples demonstrate that the tension–compression asymmetry of bi-modulus materials significantly alters the optimal topology and load transfer paths compared with conventional single-modulus assumptions. These results highlight the importance of incorporating bi-modulus constitutive models into fatigue-based design to achieve reliable lightweight structures.