Council workers and volunteers have started planting the trees across some 100 sq metres at Barkby Beach, Prestatyn.
The area lacks grass which holds the dunes together but it is hoped sand will cover the trees, giving reinforced ground on which to transplant grass.
The council said the beach was "an important area for tourism".
Denbighshire countryside officer Garry Davies told BBC Radio Wales' Country Focus programme they hoped to plant marram grass at the dunes by the end of the summer.
"It's been damaged by excessive trampling, and then once the marram disappears you get issues with wind erosion and wave erosion," he said.
"If this is left unchecked then it won't be acting as a natural sea defence."
The dunes on Barkby Beach are a Site of Special Scientific Interest and also a Special Protection area because of the important biodiversity.
They are home to the last seabird colony of little terns in Wales and are important for sand lizards, natterjack toads and rare orchids.
Mr Davies said the council hoped interpretation boards providing information and a new boardwalk system through the dunes would help people to enjoy them without causing damage.
Christmas trees are also being used in the reconstruction of sand dunes about four miles along from the beach, at Talacre in Flintshire.
The trees were collected by St Kentigern's Hospice in St Asaph, Denbighshire, which hopes its volunteers will be able to collect even more next year.
About 300 discarded Christmas trees are being dug into Denbighshire sand dunes in a bid to stop their erosion.