The　development　of　unmanned　aerial　vehicle　(UAV)　technology　has　been　advancing　rapidly　and　is　widely　applied　in　various　domains.　Compared　to　a　single　UAV,　the　multi-UAV　system,　known　as　the　UAV　Ad　Hoc　Network　(UANET),　can　collaborate　to　accomplish　complex　tasks　more　efficiently.　Due　to　the　UAVs　communicating　through　open　wireless　channels,　they　are　vulnerable　to　various　attacks.　Group　key　agreement　protocols　are　often　used　to　guarantee　the　security　of　UANET　communication.　However,　the　high　dynamics　of　UAVs　and　the　instability　of　the　wireless　links　often　result　in　certain　UAVs　temporarily　losing　connection　with　the　group,　rendering　them　incapable　of　participating　in　the　group　key　agreement　process.　Regrettably,　existing　literature　on　group　key　agreement　does　not　consider　this　issue,　which　makes　them　not　applicable　to　UANET.　To　address　this　issue,　we　propose　a　threshold　authenticated　group　key　agreement　protocol　(TAGKA)　that　can　resist　the　disconnection　of　group　members.　Then,　considering　that　the　temporarily　disconnected　UAVs　need　to　obtain　the　latest　group　key　after　the　communication　links　are　recovered,　we　design　a　group　key　recovery　protocol.　The　security　of　the　scheme　is　proved　under　the　random　oracle　model.　Moreover,　formal　security　verification　using　the　Tamarin　and　rigorous　informal　security　analysis　is　also　performed.　Finally,　we　use　NS-3　to　simulate　the　protocol,　and　the　experimental　results　show　that　our　protocol　can　be　effectively　applied　to　UANET.