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Nitrite content is one of the most important quality indices of milk. The traditional methods used to measure the nitrite content of milk are high in precision and good in repeatability, but the process is complex, the equipment is expensive, and these methods just can be used in the laboratory. To realize the detection of nitrite content of milk low in cost, fast and convenient in detection, a portable nitrite detector was developed based on the principle that a purplered azo-compound was produced when nitrite reacted with p-aminobenzenesulfonic acid and naphthylenediamine hydrochloride under weak acid conditions. Taking commercial milk as object and the NaNO2 given in Chinese national standard as nitrite, the characteristic wavelengths for detecting the nitrite content was extracted from the spectra range of 380~780nm by successive projection algorithm. Five light-emitting diodes at the wavelengths of 469nm, 500nm, 546nm, 628nm and 665nm were determined for collecting the intensity of diffuse reflected light. The developed detector consisted of a microcontroller module, a light source module, a light sensor module, a power supply, and an input and output module. Based on the intensity of diffuse reflected light at the five wavelengths, a partial least squares model was developed to predict the nitrite content of milk. The precision of the detector was verified, and the results showed that the measurement error on nitrite mass concentration ranged from -0.13mg/L to 0.07mg/L, and the average absolute error was 0.03mg/L.