Aiming at asymmetrical winding (ASW) layouts design for fractional-slot permanent magnet synchronous machine (PMSM), this article improves the conventional star-of-slot theory and proposes an effective design method of fractional-slot ASW layouts for various slot-pole combinations. In the design process, the proposed method considers the characteristic components of air-gap field time–space harmonics and the harmonic characteristic of unbalanced magnetic force (UMF). It can be used to investigate the design principles of ASW layout with a lower UMF through extracting and analyzing force vectors produced by air-gap field time–space harmonics that mainly contribute to UMF. Finally, two 39-slot and 12-pole surface-mounted PMSMs with different winding layouts are manufactured and tested. Both finite-element analysis and experimental results confirm the effectiveness of the proposed design method for double-layer ASW structures.