The fire, which heralds the arrival of spring, will be held at the ancient hill fort Castell Henllys, near Newport.
"
Celtic rituals were barbaric in nature which might be the source of the legend of the burning of the wicker man
"
Phil Bennett, manager
It harks back to a time when human sacrifices were thought to be practised at the site by the Celts.
The wicker man, which will be built on Wednesday, will be ignited to mark the festival of Imbolc.
Last year's wicker man was 15 foot high.
Manager Phil Bennett said the origins of the wicker festival were shrouded in mystery.
"Snippets of information from Roman sources suggest that Celtic rituals were barbaric in nature, which might be the source of the legend of the burning of the wicker man," he said.
"We really don't know what the truth is, although the idea that the Celts practiced human sacrifices is almost certainly true according to archaeological research."
The Castell Henllys site is unique in the UK because houses have been reconstructed in the exact holes where early settlements were built.
"In the country there are lots of reconstructed sites but these are built on the actual hill fort," said Rhonwen Owen, educational officer.
"The inside of the houses may not be the same, but the fireplaces and the actual size of the buildings is exact," she added.
The historic site, which opens fully on 1 April, is visited by about 7,000 pupils every year.
Experts' searches
Archaeologists from York University have been visiting the area for 21 years.
Last summer, experts found pickaxes, made from antler horns, dating back 2,500 years.
The Castell Henllys day is just one of a host of spring events organised by staff from Pembrokeshire Coast National Park.
Activities started on Sunday with rambles, nature watches, family arts and crafts events and geological walks being organised throughout the area this week.
The Castell Henllys event will also include activities showing what life was like 2,500 years ago.