The　basic　theory　and　mathematic　model　of　parallel　stochastic　resonance　(PSR),　chaotic　oscillator　and　difference　resonance　were　expounded.　The　correlation　analysis　was　applied　to　extract　the　same　components　from　the　output　of　difference　SR　in　the　PSR,　thereby　reduce　the　dependence　on　the　system　parameters.　When　the　chaotic　oscillator　changing　from　chaos　state　to　large-scale　periodic　state,　the　Hu　moment　invariant　will　have　a　distinct　change.　So　the　Hu　moment　invariant　can　be　used　to　automatically　quantify　the　phase　state　of　chaotic　oscillator.　The　linear　relationship　between　the　phase　diagram　size　and　signal　amplitude　is　presented　to　make　up　for　the　shortage　that　differential　oscillator　can＇t　detect　the　signal　amplitude.　Applying　the　three　nonlinear　methods　to　the　fault　diagnosis　of　mechanical　equipments,　the　weak　feature　information　of　incipient　fault　was　successfully　extracted.