To measure the grain loss of combine harvester in real-time, a grain detecting sensor using piezo-electric polyvinylidene fluoride (PVDF) films as sensitive material is presented. Signal processing circuit which is composed of a charge amplifier, band-pass filter, envelope detector and voltage comparator is designed to detect grain impact signal. Double-layer vibration isolation is incorporated in the sensor to eliminate the influence of combine harvester’s vibration. Based on the dynamic analysis of the absorbing system, the sensor response under instantaneous triangular wave impact is numerical simulated. Then, grain detecting experiments are carried out by assembling the sensor on a support stand which fixed on the vibrating cleaning sieve, the results show that with proper setting the parameters of damping material and processing circuit, the sensor can identify grain impact signal effectively from vibrating noise.