<p>Boosters play a crucial role in aviation engines. As the booster performance is significantly affected by the flow field at the fan outlet, this study experimentally investigates the radial parameter distribution at the fan outlet and numerically analyzes the impact of these parameters on the performance of a booster driven by a high-speed shaft. In ground conditions, the peak efficiency of the booster is 0.6% higher than that in standard conditions, and the pressure ratio increases by 0.15%, although the overall margin decreases by 0.75%. In high-altitude conditions, the low-Reynolds-number effect significantly degrades the performance of the booster, decreasing its efficiency by 2.4% and reducing the pressure ratio by 4.72%, with the overall margin decreasing by 8.28%. Analysis of the design parameters shows that the differences in axial parameters between ground conditions and standard conditions mainly originate from the radial nonuniformity of the inlet conditions. This nonuniformity leads to a higher corrected speed of the rotor blade roots, thereby increasing their flow capacity. However, in high-altitude conditions, the outlet flow field of the rotor is changed by the low-Reynolds-number effect, and the flow angle of the stator becomes close to a zero attack angle. This reduces the margin of the booster, weakening the blockage at the stator end region and increasing the blockage at the blade mid-span. Three-dimensional flow analysis shows that, in ground conditions, the leakage flow and shock waves at the rotor blade tips generate additional losses, whereas in high-altitude conditions, the weakened leakage flow and shock waves at the rotor blade tips lead to reduced tip losses. A lower Reynolds number enhances the separation at the blade mid-span. In ground conditions, the flow field changes of the stator are small, whereas in high-altitude conditions, a large separation zone appears on the suction side of the stator, significantly increasing the total pressure loss. This study investigates changes in the axial parameters, rotor leakage flow, shock waves, and boundary layer development at the fan inlet of the booster, and identifies the sources of losses in the stator. The impact mechanism of different inlet conditions on the performance of the booster is analyzed, and the results are expected to be of great significance in the aerodynamic design of boosters.</p>