Theoretical judgment condition of ACC extreme operation condition was achieved in terms of small deceleration and big deceleration of the proceeding car on the basis of kinematical analysis of host car and proceeding car in ACC. Considering the most dangerous operation condition in the proceeding car brakes urgently, the judgment condition of ACC extreme operation condition was confirmed in real use. And through tests，the td and acmax which were the key parameters of the condition have been obtained. The conclusion was that td equals 0.214 s and acmax equaled three different values by three typical road surfaces. Furthermore, according to the condition and typical parameters, the working area of ACC could be divided into three parts, to define the effective areas of ACC and give the driver alarm, and prevent the vehicle from entering the dangerous area after it enters the extreme area.