A method and device that identify impairments of the blood flow in an extra-corporeal blood circuit during extracorporeal treatment of the blood using an extracorporeal haemodialysis device, where the phase angle of at least one harmonic oscillation of an oscillating pressure signal that propagates in the extracorporeal blood circuit is determined. Impairments of the blood flow are detected in the extracorporeal blood circuit based on a characteristic modification of the phase angle of at least one harmonic oscillation of the pressure signal. A temporal modification of the phase angle may be compared with a predefined threshold value and an impairment is detected if the value of the modification of the phase angle is greater than the predefined threshold value. The method and device permit an early identification of a coagulation of the blood that flows through the haemodialysis unit, so that countermeasures can be initiated promptly.