Recent studies suggest a statistical anti-correlation between the X-ray variability amplitude and black hole masses in active galactic nuclei(AGN).As the established relationship was based on AGN mostly with supermassive black holes of 106 to 109 solar masses,a question remains whether it can be extended to the lower black hole mass regime.Also the anti-correlation can be derived from the power spectra density(PSD)shape of AGN.Three models of PSD have been built and predict different relationships,especially the trends in low-mass regime.In this work we present the result from X-ray variability analysis of a sample of AGN with black hole mass in the range of 105 to 106 solar masses.Our result suggests that the correlation between excess variance and black hole mass in low-mass regime is no longer simple linear relation.We compare the three models and give our suggestion on which one approaches the intrinsic PSD shape of AGN best.