In order to reveal the degradation mechanism caused by leading-edge erosion of fan blades in high-bypass-ratio turbofan engines, numerical simulations were used to investigate the differences in flow field structure and aerodynamic performance between the original and eroded transonic fan cascades under various inlet Mach numbers and outlet back pressures. The results show that: under low Mach number and high back pressure conditions, leading-edge erosion induces large-scale flow separation on the suction side, significantly distorts the flow field structure, and separation loss dominates the total pressure loss; under high Mach number conditions, the mainstream flow is dominated by flow inertia, and the overall flow field structure remains unchanged. However, the normal shock wave in the passage disperses and its action range expands, which enhances the shock wave-boundary layer interaction, intensifies turbulent dissipation and wake mixing, and ultimately increases the total pressure loss dominated by shock loss. It is concluded that the effect of leading-edge erosion on transonic cascade performance shows obvious operating-condition dependence, which is dominated by flow distortion and separation loss at low Mach numbers, while it amplifies losses through shock wave dispersion at high Mach numbers.