To monitor grain cleaning loss of combine harvester automatically and real-timely during the working process, a grain loss monitoring sensor was developed. Modal analysis was carried out under the conditions of different thicknesses and different boundaries by using ANSYS software with 304 stainless steel plate, T6 aluminum, brass plate as sensitive plates. Grain collision tests were carried out to study the relationship between the sensitive plate vibration characteristics and detecting performance by using YT-5L piezoelectric ceramic as sensitive element. Material and structure of the sensitive plate was selected. A signal process circuit which composed of voltage amplifier, band-pass filter, precision full-wave rectification and envelope detector and a control instrument which used AT89C52 microcontroller as the core chip were developed to measure the grain impact signal. The control instrument also had the alarm and communication functions. Field experimental results showed that the instrument could discriminate grain impact signal from strong interference effectively, and the grain cleaning loss ratio could be real-timely displayed, the maximum monitoring error was 2.39%. 