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Detect early performance degradation of permanent magnet synchronous motor (PMSM) is very meaningful for preventive maintenance and avoiding catastrophe failure. By fully taking into account effects of rotor flux harmonics and stator current harmonics on electromagnetic torque under actual working condition, a theoretical expression of electromagnetic torque was developed which was used to indirectly evaluate torque ripple without torsional sensor. A new health index for detecting performance degradation and its computational method were proposed by considering the different frequency domain features of three-phase stator current. Based on this the method to detect early performance degradation by monitoring changes in health index are proposed. Experimental results show that the change in the proposed health index is consistent with change in the torque ripple ratio over time and the relative deviation is no more than 10.0%. The early performance degradation of PMSM can be accurately identified by the proposed method whenever the health index exceeds the threshold.