In　typical　dynamic　soil-structure　interaction　(SSI)　problems,　the　dynamic　response　a　structure　can　be　affected　by　the　existence　of　some　nearby　structures,　which　is　sometimes　referred　to　as　the　dynamic　structure-soil　-structure　interaction　(SSSI).　This　effect　is　especially　important　in　the　earthquake　engineering　design　of　adjacent　nuclear　power　plants,　as　the　safety　risk　is　relatively　high.　However,　the　current　understanding　on　the　SSSI　of　nuclear　power　plants　is　still　insufficient.　In　this　work,　we　use　the　finite　element　method　to　investigate　the　SSSI　of　two　nuclear　power　plants　located　at　a　specific　distance　under　earthquake　excitation.　Four　nuclear　-power-plant-soil　systems　are　designed　to　account　for　the　SSI　and　SSSI　respectively,　where　the　soil　properties　are　obtained　from　drilling　data.　The　effect　of　the　SSSI　on　the　nuclear　power　plants　is　studied　by　comparing　the　dynamic　responses　of　four　nuclear　power　plants-soil　systems　in　vertical　and　horizontal　directions,　in　which　both　layered　soils　and　local　weak　interlayer　soils　are　considered.　The　results　of　numerical　study　show　that　the　presence　of　one　nuclear　power　plant　has　a　favorable　effect　on　the　seismic　response　of　an　adjacent　nuclear　power　plant,　such　as　reducing　the　displacement　response,　but　this　effect　is　limited.　In　addition,　the　SSSI　effect　is　related　to　not　only　the　soil　properties,　but　also　the　direction　of　ground　motion.　Furthermore,　the　existence　of　soft　soil　layers　complicates　the　SSSI　effect.　The　results　provide　important　insights　for　the　construction　and　expansion　of　nuclear　power　plants.