A self-adaptive passive flux-weakening permanent magnet synchronous motor (PMSM) was proposed. The magnetic conductors in the flux-weakening unit are fixed in the sliding slot at constant torque operation. The magnetic conductors begin to move outward along the sliding slot in the rotor, which can adjust the leakage flux at constant power operation according to speed automatically. The flux-weakening principle is introduced by theoretical analysis. The performance of leakage flux is analyzed. The results from theoretical analysis and simulation by finite element method verify the availability of flux weakening for the self-adaptive passive flux-weakening permanent magnet synchronous motor.