In order to measure the noise of auto shock absorbers, a test bench used to detect piston-rod vibration responses of shock absorbers and measuring analyzer named SANTS-I were developed. The vibration response data was detected by bench tests, which shows that there are high-frequency violent peaks on the sine curve of piston-rod oscillating with relative low frequency. In order to explain the interior work dynamic mechanism of shock absorbers, a schematic Micro-process Dynamic Model with 10 steps particularly divided extension and compression stroke in more detail, and dynamic differential equations for each step were presented and discussed. Furthermore, numerical simulation for the inner impacts interaction between piston and damping fluid of hydraulic shock absorber was realized by ADINA software, by the establishment of a gas-liquid two-phase finite element model. It was understood by the Micro-process Dynamic Model and numerical simulation that the cause of the abnormal noise is violent vibrations excited by inner clearance impact interaction between piston and damping oil fluid. And adherence action between valve plate and its seat, friction between piston and cylinder barrel and vaporization of damping oil fluid would intensify the impacts around the piston top dead center (TDC) and bottom dead center (BDC). Therefore, high-frequency vibration of the piston is excited and transferred to body structure of automobile, and the abnormal noise is generated.