Traditional beer freshness detection methods usually need very expensive analytical instruments and chemical reagents, which consume a lot of time and cost a lot. In this paper, spectral analysis technology is used to explore the Beer Fresh Index (BFI), which can detect the agree of beer freshness rapidly and non-destructively. Specifically, the spectrum of beer samples was collected by PSR-3500 spectrometer every 24 hours. Then, the spectral data were processed by band selection and continuum removal. The enhanced spectra showed that the depth at 842.0 nm was decreased with the increase of storage time. Therefore, the characteristic spectral index (BFI) of beer freshness was constructed based on the depth at 842.0 nm. The experimental results show that BFI value decreases gradually with the increase of storage time, which can indicate the freshness of beer well. In addition, the sensitivity of BFI to the spectral equipment was evaluated by simulating different spectral resolution and signal-to-noise ratio levels. Specifically, the data with a spectral resolution of 5～40 nm and signal-to-noise ratio of 10～60 dB are generated by using the Gauss function distribution function and the average distribution function respectively and the BFI values are calculated and analyzed. Experiments show that when the spectral resolution is less than 15 nm and the signal-to-noise ratio is less than 10 dB, the absorption feature of 842.0 nm in the spectrum are gradually concealed, and BFI is difficult to indicate the freshness of beer. However, as long as the spectral resolution is better than 10 nm and the signal-to-noise ratio is not less than 35 dB in 798～872 nm, BFI can accurately indicate the freshness of beer. The requirements of BFI for the spectrometer are not strict. To sum up, the BFI proposed in this study can accurately indicate beer freshness, serve the design of portable beer freshness equipment, and promote the application of spectral analysis technology in non-destructive detection of beer quality.