A mould aligner is used to align and lock a first mould and a second mould for moulding shell parts of a wind turbine blade. The mould aligner includes a first alignment segment configured to be attached to a first mould and a second alignment segment configured to be attached to a second mould. The second alignment segment includes a hydraulic cylinder having a barrel and a piston rod. The piston rod is axially displaceable and rotatable with respect to the barrel. The hydraulic cylinder includes a lock part having a first rod part and a locking element being provided on the first rod part. The first rod part is configured for being received in and engaged in the locking cavity of the first alignment segment. The hydraulic cylinder includes a cylinder part comprising the barrel and a second rod part having a piston.