The flux weakening mechanism of interior permanent magnet synchronous machines with segmented permanent magnets (PMs) in the rotor is studied in this paper. Air gap flux density with and without segments are analyzed and compared, based on both analytical and finite element methods. Frozen permeability technique is used to separate the PM component flux density in iron bridges at load condition, to find out the influence of demagnetizing current on flux variation. Design parameters are optimized to improve the field weakening capability. A novel explanation is proposed to explain the improvement effect of the flux weakening capability by magnet segmentation.