Online monitoring of grain loss is important to improve the efficiency of combine harvester. The traditional grain loss sensor needs to be frequently calibrated for different materials. In order to solve this problem, the grain loss sensor was developed based on PVDF (Polyvinyliden Fluoride) films. The band-pass filter based on LTC1068-200 was designed by frequency spectrum analysis of the wheat and rice grain impact response. And the dynamic voltage comparator with adaptive regulation of the threshold voltage according to the intensity of sensor signal was introduced here. Wheat and rice grain samples with different moisture content were prepared for the experiments and the results indicated the sensor had high measurement accuracy to the different samples.