This paper proposes the field separation theory for predicting the magnetic field of a quasi-Halbach array permanent-magnet motor considering the magnetization pattern and iron saturation. According to the proposed method, the air-gap field consists of a permanent-magnet field, winding current field, and equivalent saturation field. The equivalent permanent-magnet currents replacing the Halbach array with radial or parallel magnetization are introduced to obtain the linear analytical air-gap field of the permanent magnets. The winding current field relating to the slot shape and air-gap length can be directly determined using the linear analytical model. The equivalent saturation field is derived from the combination of the linear analytical model in the air gap and the magnetic circuit model in the iron region. The finite-element analysis of an 8-pole/9-slot Halbach array permanent-magnet motor and its prototype experiments are carried out to verify the effectiveness of the field separation theory, which is then used to analyze the harmonic magnetic field of Halbach array permanent-magnet motors to further improve the electromagnetic torque estimation.