This　paper　analyzes　the　over　excitation　limiter　(OEL)　simulation　model　of　the　ALSTOM　P320　V2.　excitation　system　used　by　nuclear　power　generators　in　China.　The　way　of　OEL　output　received　as　input　to　the　main　loop　is　presented　for　studying　on　the　OEL　oscillation　occurred　at　an　actual　Nuclear　Power　Plant.　The　oscillation　phenomenon　is　reproduced　in　time　domain　simulation　and　the　oscillation　mechanism　is　revealed　by　frequency　domain　root　locus　analysis.　The　concept　of　critical　gain　of　the　over　excitation　limitation　control　loop　of　ALSTOM　excitation　system　is　proposed,　and　the　tuning　range　of　the　main　loop　differential　time　constant　is　recommended.　The　feasibility　and　correctness　of　the　damping　measures　on　OEL　oscillation　are　verified　by　field　test　at　a　real　nuclear　power　unit.　©　Springer　Nature　Singapore　Pte　Ltd.　2020.