In the spectral detection of livestock products quality and safe, the thickness difference between different samples leads to the difference in the distance between the meat surface and the optical fiber probe, which has a great influence on the prediction results. Aiming at this problem, an optical sensor for nondestructive detection of meat was designed, and the quality of pork (Longissimus dorsi muscle) was detected from the bottom to the top through the glass, which eliminated the influence of sample thickness on the detection distance, and the variation law of spectral curve with the change of detection distance was also analyzed. In order to explore the feasibility of the design scheme, a test platform was set up, including spectral acquisition unit, distance control unit, light source, glass, optical sensor and computer. The quartz glass can pass through the wavelength range of 220~2500nm without absorption. And optical sensors can help collect more diffuse light from the meat. Eighteen pork samples were stored at 4℃ and taken out at different refrigeration times for spectral collection (400~1100nm) with different detecting distances in the range of 8~22mm at approximately 2mm intervals. Finally, the TVBN content of the samples were measured. After obtaining the spectral data of the samples, the methods such as first derivative, multiplicative scattering correction (MSC), standard normal variate (SNV) and first derivative plus SNV were used to pretreat the spectral data of the samples, and the partial least squares regression (PLSR) prediction model of the content of pork’s TVBN was established. The results showed that when the detection distance was 16mm, the prediction model of pork’s TVBN using the 1st DER plus SNV preprocessing method had the best prediction effect. The correlation coefficient and RMS error of correction set were 0.98 and 0.92mg/(100g), respectively, and the correlation coefficient and RMS error of prediction set were 0.97 and 1.56mg/(100g), respectively. Therefore, it was feasible to detect pork freshness with the designed optical sensor.