Abstract: Abstract: Development of remote sensing (RS), geographical information system (GIS) and global positioning system (GPS) has provided new methods for obtaining field grain yield information, which allows better description of spatial variability for grain yield. Monitoring grain yield has become an essential component in precision agriculture, which provides better guidance for grain growth and management such as variable fertilizing, irrigating and spraying. In order to further improve the monitoring accuracy of grain combine harvester, a new real-time grain yield monitoring system based on photoelectric principle was developed in this study. The system was composed of sensor module, grain yield data acquisition module, GPS module and grain yield display terminal. The sensor module included diffuse reflectance grain volume senor as key component of the system and rotating speed sensor of elevator. A model of grain mass on the scraper was established based on optical principle of photoelectric diffuse reflection effect and grain kinematic principle. Prediction model and diffuse reflectance grain yield monitoring software were embedded in the grain yield display terminal. When the elevator scraper of the combine harvester with the grain passed the diffuse reflectance grain volume sensor, the light path would be blocked intermittently. As a result, the corresponding pulse signal would be generated and meanwhile the elevator's rotating speed sensor would output the rotating speed signal. According to photoelectric principle, the size of pulse signal was proportional to the thickness of grain on the scraper. Subsequently the grain yield data acquisition module converted sensor signals into standard signals, and grain yield information including real-time grain yield and total yield, elevator rotating speed, combine harvester speed, harvest area, and longitude and latitude would be obtained and displayed on the terminal. In order to evaluate the performance of the grain yield monitoring system, both laboratory platform experiment and field dynamic experiment were conducted. For the platform experiment, an experiment platform was designed, which was composed of LED (light-emitting diode) terminal, diffuse reflectance grain volume sensor, grain inlet, elevator, elevator's rotating speed sensor and motor. The result of platform experiment showed that the rotating speed sensor of elevator had the maximum error of 1.87%, which was less than 2.00%, and the maximum standard deviation of 2.33 r/min, which indicated the sensor had a small discrete degree; the diffuse reflectance grain volume sensor had the maximum error of 3.14%, which was less than 3.50%, and both the accuracy and the stability satisfied the requirements. Field dynamic experiment included 3 parts: field experiment without loading, model calibration experiment and field experiment of wheat yield. The field experiment without loading showed that the pulse signal intensity of diffuse reflectance grain volume sensor decreased with the elevator's rotating speed increasing, the determination coefficient (R2) of output curve was 0.941 1, and the measurement error was within 4.00%. For the model calibration experiment, domestic TB60 type combine harvester was calibrated to obtain the calibration factor of 0.071, and the relationship between grain mass and thickness was gotten. The field wheat yield experiment showed that the grain yield monitoring system based on photoelectric principle was maximum error of 3.51%, which was smaller than the double-plate differential method. The system offered a wide range of grain feeding quantity and satisfied the need of field grain yield monitoring. The research provides a new method to monitor real-time grain yield, and the system is applicable to domestic mainstream models of combine harvester in China.