A spectrum analyzing method and a gingivitis evaluating device are provided. The spectrum analyzing method includes steps as follows. A diffuse reflection signal of a gingiva is calculated, and a gingiva spectrum is generated. The gingiva spectrum and a plurality of reference gingiva spectra are respectively applied with a time-series similarity calculation, and a plurality of similarity values are generated. The plurality of reference gingiva spectra correspond to various gingival indexes (GI). A minimum similarity value of the plurality of similarity values is obtained. A GI result is output according to the minimum similarity value.