Leading UK construction group, Morgan Sindall, has won the pre-construction design services contract.
The work is expected to take several months.
The Council - together with Haverfordwest High VC School - is hosting an information event in the former Sir Thomas Picton School Hall on Friday, 1st November.
Representatives from Morgan Sindall will be present to answer questions about the work from parents and the general public.
It is anticipated that construction work on the new school will start in the second half of 2020.