A cutting unit for thermoplastic pipes comprising: - a ring rotating about an axis of rotation and in which a pipe is progressively moved in a sliding fashion in such a way that a longitudinal axis of the pipe coincides with said axis of rotation; - at least one cutting arm mounted on the rotary ring and equipped with a cutting tool and having a relative portion integral with said rotary ring, said cutting arm also being movable between an operating position, wherein said cutting tool is engaged with said pipe for exerting a cutting action, and a rest position, wherein said cutting tool is disengaged from said pipe; - at least one distance measurement sensor located on said rotary ring and configured for acquiring, during a rotation of said rotary ring, a plurality of values representing a radial distance between said at least one measurement sensor and an outer surface of said pipe, said distance being measured on a measurement plane transversal, in particular perpendicular, to said axis of rotation; - a processing and control unit operatively connected to said at least one distance measurement sensor and configured to receive said plurality of representative values measured and to calculate, based on said representative values measured, at least one parameter representing a peripheral profile of said pipe on said measurement plane, wherein said processing and control unit is configured to send to the actuator of the cutting arm a signal of movement towards, or away from, said pipe as a function of a comparison between the value measured by said first sensor and a expected positioning value, said expected positioning value being calculated on the basis of values representing the radial distance acquired by the distance measurement sensor. - a ring rotating about an axis of rotation and in which a pipe is progressively moved in a sliding fashion in such a way that a longitudinal axis of the pipe coincides with said axis of rotation; - at least one cutting arm mounted on the rotary ring and equipped with a cutting tool and having a relative portion integral with said rotary ring, said cutting arm also being movable between an operating position, wherein said cutting tool is engaged with said pipe for exerting a cutting action, and a rest position, wherein said cutting tool is disengaged from said pipe; - at least one distance measurement sensor located on said rotary ring and configured for acquiring, during a rotation of said rotary ring, a plurality of values representing a radial distance between said at least one measurement sensor and an outer surface of said pipe, said distance being measured on a measurement plane transversal, in particular perpendicular, to said axis of rotation; - a processing and control unit operatively connected to said at least one distance measurement sensor and configured to receive said plurality of representative values measured and to calculate, based on said representative values measured, at least one parameter representing a peripheral profile of said pipe on said measurement plane, wherein said processing and control unit is configured to send to the actuator of the cutting arm a signal of movement towards, or away from, said pipe as a function of a comparison between the value measured by said first sensor and a expected positioning value, said expected positioning value being calculated on the basis of values representing the radial distance acquired by the distance measurement sensor.