Because　of　simple　shape　and　easy　production,　equilateral　frustum　shells　are　often　used　as　initial　shells　for　the　dieless　hydroforming　of　small　diameter　spherical　vessels.　But　in　the　forming　process,　the　shell　equator　tends　to　withdraw　inwards　and　creates　wrinkles,　which　badly　affect　the　vessel　quality.　The　rigid　plastic　finite　element　analysis　is　employed　to　simulate　the　hydrobulging　process　of　the　shell　under　internal　pressure,　and　then　some　modifications　for　the　initial　shape　of　the　frustum　shell　are　presented　to　restrict　the　withdrawing　of　equator　and　to　prevent　it　from　wrinkling.　The　simulations　results　show　that　the　modified　frustum　shell　is　simple　in　shape　and　effective　in　anti-wrinkling,　and　has　the　capability　to　obtain　a　really　round　vessel,　which　is　not　possible　with　an　equilateral　frustum　shell.