In this paper, a new method for quantitative determination of fat content in goat milk at room temperature (25°C), based on the annular photoelectric sensor system, has been developed. The measurement system consists of an annular photoelectric sensor, thermoelectric cooler (TEC), amplifier, A/D converter, microprocessor and other components. Based on Mie theory, the diffuse reflection light intensity is adopted as the optical parameter representing the fat content in goat milk. In this way, the standard model of the system can be established and loaded into the processor to realize data acquisition, processing and output. The proposed method has been tested on goat milk samples with variable fat content in the prediction experiment. The results show that the fitting equation is y=–0.366x+0.295 (R²=0.987) and measurement errors are within ±3%, which indicates that the presented method has high prediction performance and can measure the fat content in goat milk accurately and in real time.