Device for moulding thermoplastic containers by blow-moulding hot preforms, comprising a mould (1) and blow-moulding means (2) comprising a bell nozzle (8) which is axially displaceable between a raised position and a blow-moulding position in abutment against the mould; the driving means comprising mechanical means (12) with a cam follower roller (13)/cam (14) and pneumatic means (16) with a piston (18) integral with the upper end of the nozzle and sliding in a fixed enclosure (17), defining therein an upper chamber (19) connected to a source (24) of control fluid and a lower chamber (20) connected to this source and to the upper chamber via a solenoid valve (26); the upper face of the piston has a surface area which is greater than that of its lower face and that of the bell (9) of the nozzle.