Plant (2) for the production of slabs made of composite stone material from a mix (M) containing granules of stone or ceramic or glass material and a binder, comprising at least one temporary support or mould (S), a distributor for the mix (M) designed to distribute a layer of mix (M) on the temporary support or mould (S), a station (4) for performing compaction by means of vacuum vibro-compression of the mix (M) arranged on the temporary support or mould (S) and a station for hardening the mix (M) so as to form the finished slabs. The plant (2) comprises a device (1) positioned downstream of the distributor and upstream of the compaction station (4) and comprising means (16) which act on the top surface of the mix (M) for pre-compaction and levelling of the mix (M) arranged on the temporary support or mould (S). The invention also relates to a method for the production of slabs of composite stone material from a mix (M).