Dual three-phase permanent magnet synchronous motors (DTPMSMs) are widely used in fields such as new energy vehicles, rail transit, and ships due to their small output torque rippling and high reliability. However, long-term overload, uneven magnetic field distribution, overvoltage, insulation damage, and other factors can easily lead to inter-turn short circuits in windings, posing significant safety hazards. It is essential to study the diagnosis method of inter-turn short circuit faults in DTPMSM in order to improve their safety and reliability. Firstly, this article derives the mathematical model of inter-turn short circuit (ITSC) faults in the rotating coordinate system of a DTPMSM, revealing the characteristics of winding ITSC faults. Secondly, a sliding mode observer (SMO) is used to observe the back electromotive force (EMF), constructing a fault indicator by converting the residual into a direct flow to diagnose the fault. Finally, the simulation results show that the method studied in this article can effectively diagnose whether there is a fault in the DTPMSM.