The correct K value of an undulator is an important parameter to achieve lasing conditions at free electron lasers. The accuracy of the installation of the undulator in the tunnel is limited by the accuracy of the instruments used in surveying. Moreover, the position of the electron beam also varies depending on its alignment. Another source of misalignment is ground movement and the resulting change in the position of the tunnel. All this can lead to misalignment of the electron beam position relative to the center of the undulator gap up to several hundred microns. That, in turn, will lead to a deviation of the ΔK/K parameter several times higher than the tolerance requirement. An automated method of aligning the middle plane of the undulator, using a K-monochromator, was developed and used at European XFEL. Details of the method are described in this article. The results of the K value measurements are discussed.