Spectral gain narrowing is one of the key factors affecting broadband amplification of ultrashort pulses. In this paper, the spectral gain characteristics in broadband amplification are studied theoretically and experimentally by using the characteristic of Nd,Gd:SrF₂ crystal, i.e. the emission spectrum that has a certain width at the higher stimulated emission cross section. Through the numerical simulation, the evolution law of output spectrum of the laser gain medium under different spectral gain lineshapes and different gain values is studied in detail. Theoretical calculation shows that the spectral gain is narrowed obviously with the increase of gain value of the traditional Gaussian emission spectrum, and that increasing the spectral bandwidth at the maximum stimulated emission cross section can obviously suppress the spectral gain narrowing. Furthermore, the spectral gain narrowing characteristics of the Nd,Gd:SrF₂ crystal are studied experimentally. The Ф 13 mm× 150 mm Nd,Gd:SrF₂ crystals are used as the gain medium which are pumped by flash lamps in the experimental study. The experimental results show that the output spectra of Nd,Gd:SrF₂ crystals are not obviously narrowed when the full width at half maximum (FWHM) of spectral width of the input laser is 5 nm and the gain is 140 times. The experimental results are consistent with the theoretical calculation and analysis. The crystal can work normally at a repetition rate of 0.2 Hz and 1.0 Hz in the experiment, but due to the influence of thermal effect, the gain will decrease to a certain extent with the increase of pump energy and repetition rate. The research results lay the foundation for the application of fluoride crystal in broadband chirped pulse amplification.