Preventive conservation requires suitable and stable temperatures and relative humidity (T & R.H.) in a museum environment. This paper is about research on a new evaluation method for T & R.H. stability. Museum environment monitoring data of temperatures and relative humidity were converted into spectrum maps using fast Fourier transform, and the T & R.H. stability was analyzed by the interpretation of the spectrum maps. The merits and demerits of spectrum maps of T & R.H. were analyzed in detail, and the fluctuation characteristics and the stability of T & R.H. in exhibition halls and showcases as well as the control effect were evaluated by the interpreted spectrum maps. This paper provides a new and effective method for evaluating the T & R.H. stability of a museum environment.