Knock Sensor is installed on the cylinder wall of the engine, which is adapted to detect whether a knock occurs in the engine, so that the spark advance angle can be adjusted correspondingly. A vibrating plate is arranged inside the sensor. When a knock occurs in the engine (with a frequency of about 7-8 KHz), resonation appears on the vibrating plate, and this vibration acts on a piezoelectric element, the piezoelectric element generates a voltage. When the engine is in a working state of acceleration or high load and a slight knock occurs, the sensor inputs a knock signal to an ECU (Electronic Control Unit). When the ECU receives the knock signal from the Knock Sensor, it gives instructions to reduce the spark advance angle, until the knock ceases.