The　optimal　support　timing　for　the　tailrace　tunnel　of　a　certain　HSP　was　determined　through　the　load　release　method.　The　results　show　that　when　the　load　release　rate　(LRR)　of　the　surrounding　rock　(SC)　reaches　80%,　the　deformation　increment　at　the　measuring　points　increases　significantly,　and　this　is　the　best　support　timing　for　this　deep-buried　tunnel.　Taking　the　UP　of　this　hydropower　station　project　(HSP)　as　an　example,　the　distribution　laws　of　the　deformation　and　stress　of　the　SC　of　the　underground　powerhouse　(UP)　under　the　condition　that　the　excavation　LRR　is　80%　are　obtained　through　the　typical　calculation　example　of　three-dimensional　nonlinear　FEA.　The　results　show　that　stress　concentration　is　prone　to　occur　at　the　top　arch,　bottom　plate,　and　especially　at　the　corners　of　the　chamber.　Therefore,　enhanced　support　measures　should　be　taken　during　the　subsequent　support　construction.　©　2025　IEEE.