In a game moved from Llandovery to Ystrad due to the stormy weather and conditions in West Wales, it was the hosts that took the early lead with two tries from wing Wellington.
However, the game started to swing towards the visitors after a penalty try was awarded after 24 minutes to Scarlets and Scott Evan was yellow carded.
Scarlets took a 17-12 lead to the interval after a converted try from Kieran Hardy just before the break.
The second half saw Dragons A twice reduced to 14 men – with Taine Basham and James Thomas sin binned – and Scarlets took full advantage to run in six more tries to book a place in the Celtic Cup final.
03m Wellington t 5-0
08m Wellington t Botica c 12-0
13m Wilson p 12-3
24m Penalty Try 12-10
32m Hardy t Wilson p 12-17
41m Wilson p 12-20
57m Evans t Wilson c 12-27
60m Hughes t 12-32
65m Smith t Wilson c 12-39
72m Baldwin t Wilson c 12-46
74m Smith t Blommetjies c 12-53
76m Conbeer t 12-58
Dragons A (v Scarlets A):Carwyn Penny (Brookson 73), Calvin Wellington, Tom Hoppe, Anuerin Owen, Deon Smith (Smith 40), Jacob Botica (Kelly 48), Dan Babos (Buckland 67); Scott Evans (Reynolds 55), Zac Hewlett, Chris Coleman, James Thomas, Max Williams (Scragg 55), Ben Fry, Lennon Greggains (c), Taine Basham
Replacements: Brodie Coghlan, Josh Reynolds, Luke Yendle, Ed Scragg, Rob Brookson, Dafydd Buckland, Llew Smith, Will Kelly