For the problem of cogging torque existing in surfacemounted permanent magnet (SPM) motor, a method of single magnetic pole lengthening was proposed to reduce the cogging torque. The analytical expression of the cogging torque was derived by the analytic method when the single magnetic pole of the motor was lengthened and the variable B_r(vz) which had an important influence on the cogging torque was obtained through Fourier analysis. The magnetic pole lengthening factor η was introduced and the way to choose the value of η to reduce the cogging torque was studied. The availability of the method was validated by finite element simulation. It was found that the cogging torque of SPM motor could be effectively reduced when the B_r(vz) related to the key harmonic components of the cogging torque was reduce to zero by reasonably selecting the magnetic pole lengthening factor.