Chapel Bay Fort and Museum at Angle will open on 3 April after years of restoration work.
The coast artillery fort was completed in 1891 and is the earliest known fort in the world constructed in concrete.
Twelve Victorian forts were built in the 19th Century to defend the Milford Haven Waterway.
On display will be the biggest gun and shell in Wales; a 25-pounder gun howitzer, a 5.5 inch gun and a 10 inch, 18 ton rifled muzzle loading gun.
There will also be displays on World War One, bomb disposal, artillery, small arms, the telephone exchange and the Royal Artillery forge and workshop.
A Victorian fort and museum in Pembrokeshire is opening to the public for the first time and is appealing for volunteers to help run it.