Planetary transmission system has been widely used in modern industry, and study on planetary gearbox degradation feature analysis method has important significance for mechanical system prognostics and health management (PHM). In the light of the degradation characteristic of planetary gearbox, root means square (RMS) spectrum is proposed based on the signal processing method and composition way of Kurtogram. RMS is calculated as the characteristic index instead of kurtosis in RMS spectrum. As a result, RMS spectrum presents the RMS changes of every frequency band of time domain and frequency domain in the system degradation process. The principles and applications of RMS spectrum and RMS normalized spectrum are described by bearing experimental data examples. A detailed study of planetary gearbox degradation process characteristics analysis based on the RMS spectrum is expressed in the case study. There are some conclusions obtained from the case study, such as the RMS changes are not the same for different frequency bands; Some frequency bands can stably reflect degradation process, whilst some frequency bands are sensitive to the acceleration of the degradation stage. These conclusions are very meaningful to the extracting planetary gearbox degradation feature index in degradation prediction.