This paper presents a methodology for calculating the operability and performance of split-flow fans with inlet distortion. A one-dimensional stage-by-stage model has been developed to include parallel compressor theory. The model is capable of providing a detailed definition of the circumferential and radial flow fields. The low-pressure fan is divided to three radial divisions, near-hub, near-tip, and mid-span. The characteristics of each radial segment are calculated by using fan radial profiles. The stable and unstable operations of a split-flow fan, with and without inlet distortion, have been investigated in this study. For the clean inflow surge was triggered by the core compressor, whereas the fan spoiled sector was found to cause the compression system to surge for the distorted case.