The empirical mode decomposition (EMD) method has inherent defects because of a recursive decomposition. This paper introduces variational mode decomposition (VMD) into knock detection field, based on variational principle. Compared with the EMD, the VMD has better efficiency and accuracy, and more robust, which is better for knock detection in the vibration signal with strong background noise. In this case, this paper proposes an adaptive selection of VMD's level number using the center frequency of different components, because the VMD method needs presetting the numbers of modal components. Decomposing a signal by the VMD in a low level, and increasing the decomposition level one by one are available till the center frequency of different components meet the predefined threshold, in whichthe best decomposition results can be obtained. The method is proved by the verification and comparison of experimental data.