With the recent advances in sensors, electronics and computational processing power, automated technologies for combine harvesters have been made possible in part and there is an urgent need to develop a system which could monitor the clean loss in real-time. In order to monitor grain clean losses of combine harvester real-timely during the working process, distribution of grain cleaning losses on rear of cleaning sieve was studied, and the mathematical model between total losses of grain cleaning and grain volume tail on different regions was developed. The optimal mounting position of grain loss monitor sensor on combine harvester was confirmed. The experiment results showed that there was no significant impact on distribution of grain clean loss mass ratio with fan speed of 1200~1400r/min under the significance level α=0.05. The grain losses monitoring sensors, which utilizing type YT-5L piezoelectric ceramic as sensitive element, was installed on combine harvester with a vertical distance of 300mm between central line and tailing screen and a angle of 45°. Meanwhile, the grain harvesting filed test was carried out by using the mathematical model. The results showed that the established mathematical model had a good reliability, the maximum relative monitoring error of the grain cleaning loss detecting system was only 3.26%, which was relatively less than checked manually when harvested rice.