Earthquake　action　is　multi-dimensional　and　random.　The　anti-seismic　capability　of　beam-column　joints　in　reinforced　concrete　(RC)　frame　structures　under　arbitrary　horizontal　direction　loading　may　be　lower　than　the　design　capability　for　unidirectional　earthquake　actions.　To　date,　detailed　calculation　methods　for　the　shear　capability　and　shearing　performance　of　joints　in　RC　frames　under　arbitrary　horizontal　direction　loading　have　not　been　reported.　For　this　reason,　in　this　work,　the　shear　mechanism　of　beam-column　joints　in　an　RC　frame　under　arbitrary　horizontal　direction　loading　is　analysed.　This　study　shows　that　two　oblique　compression　zones　form　at　the　corners　of　the　joint　when　a　synthetic　shear　force　is　imposed　on　the　joint,　and　an　oblique　compression　strut　is　formed　in　the　core　area　of　the　joint,　which　is　different　from　the　shear　mechanism　of　a　joint　under　unidirectional　loading.　Furthermore,　a　shear　capacity　calculation　model　is　established　based　on　the　strut-and-tie　model.　The　predicted　values　of　the　shear　capability　in　this　work　are　in　good　agreement　with　the　reported　experimental　results.