The evolution of a local helical perturbation and its stability property for arbitrary magnetic shear configurations are investigated for the case of in cylindrical geometry. An analytic stability criterion has been obtained which predicts that a strong magnetic shear will enhance the instability in the positive shear region but enhance the stability in the negative shear region. The perturbations with the poloidal and toroidal perturbation mode numbers m/n = 1/1 is most unstable due to the stabilizing terms increasing with m. For m/n = 1/1 local perturbations in the conventional positive magnetic shear (PMS) configurations, a larger qmin exhibits a weaker shear in the core and is favourable to the stability, while in the reversed magnetic shear (RMS) configurations, a larger q0 corresponds to a stronger positive shear in the middle region, which enhances the instability. No instabilities are found for m≥2 local perturbations. The stability for RMS configurationis not better than that for PMS configuration.
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