Abstract: Aiming at the problem of the wear condition monitoring of the roller in the control rod drive mechanism in reactors, the concept of complexity is introduced. As one of the complexity, the singular spectrum entropy is used to recognize the wear condition. After the reconstruction phase space of the observed data, the singular is calculated. Combined with the concept of entropy of information, the singular spectrum entropy is defined. The singular spectrum entropy is sensitive to the uncertainty of the observed data and shows the change of the components in the frequency domain which is related to the wear condition. By the experiment of the control rod drive mechanism in its whole life, it is found that the high-frequency components increase and the low-frequency components change little, with the running time increasing. The experiment proves that the method of singular spectrum entropy is quite valid in wear condition monitoring and recognition.