In order to analyze the effect of sensor structure on the character of grain impulse response, 3-D models of grain detecting sensor were established and their vibration modal were simulated by using software SolidWorks. It was found that the dimension of the upper panel was an important factor which influenced the modal nature. With the increasing of upper panel width b , its natural frequency f monotonically increased, and its relative deformation ratio γ quickly decreased at first and then decreased to a constant value gradually. With the increasing of thickness h , monotonic increasing tendency of f and γ was received. Then, experiments were carried out by assembling the sensor on a support stand which fixed on the vibrating cleaning sieve, the results showed that the sensor can identify grain impact signal effectively from vibrating noise. The time band of grain impact signal acquired using sensor with b of 30mm width was less than 2ms which was much small than the signal acquired using sensor with b of 120mm.