There is flow separation in the compressor intermediate casing of a turbofan aero engine with a high bypass-ratio. The loss and the distortion of outlet flow field caused by this have important influence on the performance of aero engines. An experimental study on the effect of inlet distortion on the aerodynamic performance of the compressor intermediate casing of a high bypass ratio turbofan aero engine is conducted by mounting a 20% duct-height spoiler at the hub endwall to simulate radial inlet distortion. The results indicate that the total pressure recovery and loss coefficient at the inlet due to inlet distortion decreases by 0.684% and increases by 9.74%, respectively, compared with the uniform inlet, and severe loss of total pressure and total temperature are detected below 40% of the overall duct height. At the outlet section, the total pressure fluctuates violently and a low pressure region appears below 40% height. The total temperature at the outlet is evidently higher than the uniform inflow because of flow mixing, and velocity at the outlet decreases obviously. After the flow with inlet distortion passing the intermediate casing, the distortion of the total temperature and total pressure decreases and distributes more uniformly. Compared to the variation of inlet total pressure distortion which is up to 30%, the one at the outlet reduces to about 10%, and the intermediate casing has an obvious improving effect on the development of inlet distortion along the flow direction.