Protein content is one of the most important quality indicators of milk. The traditional methods used to measure protein content in milk have high precision, but the measurement procedures are tedious and timeconsuming, and the measurement equipments are expensive. In addition, these methods can not be used in field or insitu measurement. Therefore, developing a kind of portable milk protein content detector with simple operation and low cost, which could be used insitu measurement, would be appreciated by milk producers, processers and consumers. Since the hydrophobic grouping of Coomassie brilliant blue G-250 has strong affinity for protein in acids, a portable milk protein content detector was developed based on light transmission. The hardware of the detector consisted of the modules of single chip microcomputer, light source, light sensor, input/ouput and power. The software was developed by C51 language, and was made up of main function and several subfunctions to realize light collection, data preprocessing, protein content calculation and display. Raw fresh milk was used to study the relationship between intensity of transmitted light and protein content. It was found that the intensity of transmitted light was decreased with the increase of protein content, and they had strong linear relationship with the coefficient of determination of 0.872. The validation experiment on the developed detector showed that contrasted with Kjeldahl method, the absolute measurement error of the developed detector was within the range of ±0.08g/(100g), and its mean absolute error was 0.05g/(100g). In addition, the measurement result could be given in 2s.