In order to quickly and accurately calculate fault tolerant operation capability of multiphase permanent magnet synchronous motor (PMSM) at asymmetrical fault conditionan analytical model of no-load and on-load magnetic field of multiphase inner rotor surface permanent magnet synchronous motor (SPMSM) under asymmetrical fault condition is proposed by the Fourier series analysis method in the two-dimensional polar coordinate system. The performances such as air gap magnetic densityno-load back electromotive force (EMF) and electromagnetic output torque are calculated accurately. A 12-phase inner rotor SPMSM with 44-poles and 48-slots for ship is designed and manufacturedand theaccuracy of proposed model is validated by finite element analysis and test. On this basisin order to improve the fault tolerance ability of multiphase PMSMa fault tolerant control strategy for open circuit fault of one phase winding of multiphase motor drive system is established with minimum-phase current as the optimization goal. Taking the performances such as electromagnetic output torque and torque ripple as evaluation indexesthe fault tolerant performance of 12-phase SPMSM is analyzed to further improve the reliability of motor drive system.