This　paper　focuses　on　the　nonlinear　vibrations　of　stay　cables　and　evaluates　the　dynamic　characteristics　of　stay　cables　by　using　the　nonlinear　enhanced　MECS　approach　and　the　approximate　approach.　The　nonlinear　enhanced　MECS　approach　is　that　both　the　girder-tower　vibrations　and　the　cable　vibrations　including　parametric　cable　vibrations　are　simultaneously　considered　in　the　numerical　analysis　of　cable-stayed　bridges.　Cable　finite　element　method　is　used　to　simulate　the　responses　including　the　parametric　vibrations　of　stay　cables.　The　approximate　approach　is　based　on　the　assumption　that　cable　vibrations　have　a　small　effect　on　girder-tower　vibrations,　and　analyzes　the　local　cable　vibrations　after　obtaining　the　girder-tower　responses.　Under　the　periodic　excitations　or　the　moderate　ground　motion,　the　differences　of　the　responses　of　stay　cables　between　these　two　approaches　are　evaluated　in　detail.　The　effect　of　cable　vibrations　on　the　girder　and　towers　are　also　discussed.　As　a　result,　the　dynamic　characteristics　of　the　parametric　vibrations　in　stay　cables　can　be　evaluated　by　using　the　approximate　approach　or　the　nonlinear　enhanced　MECS　approach.　Since　the　different　axial　force　fluctuant　of　stay　cables　in　both　ends　of　one　girder　causes　the　difference　response　values　between　two　approach,　it　had　better　use　the　nonlinear　enhanced　MECS　approach　to　perform　the　dynamic　analyses　of　cable-stayed　bridges.