Because existing human experience diagnosis method and apparatus for auxiliary diagnosis method are difficult to diagnose quickly engine knock. Atomic absorption spectrometry was used to detect the automobile engine knock in an innovative way. After having determined Fe, Al, Cu, Cr and Pb content in the 35 groups of Audi A6 engine oil whose travel course is 2 000~70 000 kilometers and whose sampling interval is 2 000 kilometers by atomic absorption spectrometry, the database of primary metal content in the same automobile engine at different mileage was established. The research shows that the main metal content fluctuates within a certain range. In practical engineering applications, after the determination of engine oil main metal content and comparison with its database value, it can not only help to diagnose the type and location of engine knock without the disintegration and reduce vehicle maintenance costs and improve the accuracy of engine knock fault diagnosis.