Abstract: The ratio of the separation loss was an important indicator to measure the operating performance of a combine harvester, and it also was an important criterion to adjust the relevant operating parameters. It was hard to monitor the separation loss directly for a combine harvester when harvesting high-yielding rice. To monitor the separation loss in real time, this paper promoted a new method to monitor the separation loss. The method mainly included the following steps: draw distribution functions of separated grain in an axial direction of a threshing rotor in a laboratory test-rig and select a proper detecting area under the separating concave; then establish a mathematical model among relevant variables; develop a grain detecting sensor which could discriminate free grains from MOG; and fix the sensor on a combine harvester, based on the detecting mathematical model to calculate grain separation loss in real time. To achieve this purpose, a threshing-separation experiment was carried out in an axial threshing-separating-cleaning test-bed, which selected Wu "2645" rice as test materials. In addition, the vertical and horizontal distribution of the mixed material under the longitudinal-axial drum was studied with different feeding quantity. More detailed comparative analysis and research was carried out on different feeding quantity, deriving the separation probability model along the radial and axial direction of the longitudinal axial of the flow threshing drum, and establishing an indirectly monitoring mathematical model of separation loss. The accuracy of the grain loss monitoring sensor was the key to accurately measuring the grain flow in the monitoring area. To obtain the grain detecting signal effectively, high-sensitivity receiver material piezoelectric ceramic YT-5 and PVDF films were selected as sensitive components, and a signal process circuit which was composed of voltage amplifier, band-pass filter, precision full-wave rectification, envelope detector to measure the grain impact signal and a secondary instrument which used AT89C52 microcontroller as the core chip were developed to acquire the grain impact signal, a collision signal that had been studied according to comparative collision test; an ellipsoid particle model was established according to the physical properties of rice grain and used the discrete element method, which used Hertz-Mindlin to model as a contact mechanics model, to simulate the impact process of a grain with a detection sensor, which provided a basic reference for selecting the sensitive plate, and one-degree freedom vibration isolation was incorporated in the sensor to eliminate the influence of the combine harvester's vibration. The grain loss monitoring sensor was installed on the combine harvester utilizing the indirect monitoring method which was promoted in this paper to detect separation loss in the field working process. Field test results showed that the separation loss monitoring method was feasible, and the performance of the grain loss monitor sensor was stable. The biggest measurement relative error of the separation losses was 3.03% when harvesting rice. The study of this paper realized real-time monitoring grain separation loss, which laid a good foundation for a practical engineering application.