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In order to monitor grain loss of combine harvester real-time during the working process, a grain loss monitoring system was designed. PVDF piezoelectric films were used as the sensitive element, and signal process circuit which is composed of voltage amplifier, band-pass filter, precision full-wave rectification, envelope detector was constructed to measure the grain impact signal. A double-layer vibration isolator was proposed to eliminate the vibration influence of combine harvester. Grain impact performance tests were carried out within the impact velocity of 1.1~2.6m/s，which showed that the peak voltage of the impact signals within 2~4V, and peak voltage increased with increasing impact velocity.Using AT89C52 microcontroller as the core chip, a secondary instrument was developed to acquire the grain impact signal which is output from detecting sensor. Field experimental results showed that the instrument could identify grain impact signal effectively, and the grain separation loss ratio and cleaning loss ratio could be real-time displayed, the system also had the alarm and communication functions.