Objective to research the mechanical style(compressive or tensile force)of the key site of brain tissue in brain deceleration impact.Method a transparent physical brain model with air bubbles was built and loaded on an upright brain decaler ation impacting expedmentel platform.Then,the moveable platform was made a free fall from a height of 400mm and impacted on the fixed platform,and the whole deceleration impacting process was recorded by a high-speed video camera.Using the serial pictures analyzing Software,the length change of the long ads(vertical to the impacting direction)and the short axis(in the impacting di-rection)of the air bubbles were analyzed and calculated.Result the length change of the long axis of air bub-ble with in site coup was smallerthan the absolute value of that of the short axis;while with the air bubble in the contrecoup site,the length change of the long axis was bigger than the absolute value of the short axis.Conclusions the results showed that the air bubble in the coup site mainly suffered from the tensile force vertical to the impacting direction and the air bubble in the contrecoup site mainly suffered from the compres-sive force in the impacting direction.Since the propeny of tensile resistance of the brain tissue is inferior to the property of compressive resistance of the brain tissue, the injury is often easier to occur in the contrecoup site than in coup site. The results were of significance to the research of biomechanical mechanism, diagnosis and prevention of the brain deceleration impacting injury.