The　upper　and　lower　limits　of　detected　frequency　for　differential　resonator　were　obtained　by　experimental　method.　The　detected　upper　limit　frequency　only　need　to　satisfy　the　sampling　theorem　while　the　detected　lower　one　is　related　to　sampling　frequency.　Under　the　condition　of　certain　sampling　frequency,　the　lower　detecting　frequency　of　differential　resonator　was　about　1/650　of　sampling　frequency.　When　the　sampling　frequency　was　too　high,　whether　converging　to　the　pole　or　the　polar　ring,　the　phase　diagram　of　differential　resonator　would　present　as　a　pie.　Applying　this　method　to　the　high　speed　rolling　mill　fault　diagnosis,　the　phase　diagram　of　the　vibration　signals　changed　from　pole　to　polar　ring,　which　implied　the　running　state　from　normal　to　incipient　fault,　thereby　providing　an　effective　technology　for　condition　monitoring　and　fault　diagnosis　of　mechanical　equipment.