Abstract: In order to monitor the separation loss of combine harvester automatically in the field work process, an indirect grain separation loss monitoring method was promoted in this paper. By analysis the distribution of mixed material under the vertical and horizontal direction of the longitudinal-axial drum on different feeding quantity, mathematical model among the grain quantity of monitoring regional , feeding quantity and total separation losses was established; installed the grain loss monitoring sensor which utilize piezoelectric polyvinylidene fluoride(PVDF) film as sensitive material in the selected area under the concave to fulfill the task of monitoring separation loss according to established mathematical model. Field tests results showed that the measurement errors of grain separation loss were less than 3.4% when harvesting rice.