Protein content is one of the most important quality indicators of milk. The traditional methods used to measure the protein content of milk have the advantage of high precision, but the measurement procedures are tedious and timeconsuming, and the measurement equipment are expensive. In addition, these methods cannot be used in filed or insuit. Therefore, developing a kind of portable milk protein content detector which is simple in operation and low in cost, and could be used insuit measurement, will be appreciated by milk producers, processers and consumers. Since the values of electric parameters of milk are affected by protein content, a portable protein content detector over the frequency range of 1~100MHz was developed. The hardware of the detector consisted of a STM32 single chip microcomputer, a frequency sweep signal source module, a detection module, a signal processing module, and an input and output modules. The software, developed under the development environment of MDK 5.0 by using C language, realized the functions of initialization, frequency sweep, data sampling, serial port transmission, key processing and result display. Fresh raw milk were used as samples to analyze the relationship between the protein content and the obtained data, i.e., the amplitudes of input and output signals, and the phase difference between the input and output signals. The partial least squares model was established to predict the protein content based on obtained data. The results showed that the model had a determination coefficient of 0.835, the absolute measurement error of -0.11~0.12g/(100g), and the average absolute error of 0.01g/(100g). Moreover, the measurement could be realized in 2 min. The developed detector made the detection of protein content of fresh raw milk realized quickly and effectively.