Method for assembling a turbomachine (1) by means of a device (10), the turbomachine comprising at least two modules (3) which are assembled by the insertion of a shaft of one of the modules into a housing of the other of the modules, the device comprising: means (11) for supporting a first of the modules, means (20, 21) for suspending a second of the modules and for moving this second module along an axis of movement (Z), a laser beam emitter (30) intended to be fixed to the said first module and configured to emit a laser beam (31) that coincides with a longitudinal axis (X) of this first module, and a target (40) intended to be fixed to the said suspension and movement means so that it can be moved along the said axis of movement, and so that in at least two axial positions on this axis which are distant from one another, a spot from the said laser beam is located at the centre of the said target, the method being characterized in that it comprises the steps of: a) positioning the said first module (3) on the said support means (11), b) fixing the said target (40) to the said suspension and movement means, c) determining a first axial position of the said target, for which position a spot from the said laser beam (31) is positioned at the centre of the said target, d) moving the said target along the said movement axis (Z), and e) determining a second axial position of the said target, for which position a spot from the said laser beam is located at the centre of the said target, so as to validate the parallelism between the said longitudinal axis (X) of the said first module and the said movement axis.