Cup holders Llandaff North put the first points on the board after just five minutes when an intelligent kick from centre Vikki McGrath was collected by Abby King to score in the corner.
It was a tough game and Caernarfon's No 8 Kylie Wilson made a couple of dangerous runs through the mid field but Llandaff North increased their lead through tries from scrum half Amy Day and centre Jess Kidley.
Caernarfon winger Manon Williams ran in two tries for the Northern club but in the last minute Llandaff North's Kate Newton powered over the line to put the game out of reach.
Tylorstown had a convincing win in their semi final, beating Bala 39-5. The two teams will meet in the Cup final on the 11th May at Pontypridd RFC (2pm KO).
Llandaff North are on course to retain their National League title for a second year having already won the South Division 1 title. They will play Caernarfon again, the winners of North Division 2 on the 18th May in the National league final.