We have studied the spectral (UV-vis absorption and fluorescence) and thermally spectral stability of seven fluorene-based blue-light-emitting polymers in film states. These polymers have different side chain and backbone structure. Spiro-functionalization at the C-9 bridge position of fluorene unit could significantly improve the emission spectral quality (narrower spectrum and shorter tail extended to longer wavelength direction) and thermally spectral stability of 9,9-disubstituted polyfluorene derivatives. A glass transition temperature dependence for excimer emission in the polymers was demonstrated, and the improvement of thermally spectral stability by the spiro-functionalization is attributed to the increase of glass transition temperature. The backbone structural modification for 9,9-disubstituted polyfluorenes by alternatively inserting substituted phenylene units could provide blue emission with the spectral quality and thermally spectral stability even better than spiro-functionalized polyfluorenes, and no glass transition temperature dependence for excimer formation was observed in the backbone-modified polymers. The spectral properties of the polymers are dependent on the substitution on the phenylene ring. Thermotropic liquid crystallization was observed in the polymers bearing long alkoxy substituents. The good thermally spectral stability of the polymers is attributed to the poor planar configuration of the backbone and the efficient separation of the side chains on phenylene units for backbones.