Abstract: The relationship between the sensitivity of diesel to flow improver and some factors such as n paraffin content and distribution of n paraffin in diesel is studied. The function of diesel flow improver and content of n paraffin have the best matching point. The fewer average carbon number of n paraffin in diesel is, the lower cold filter plugging point is. When the distribution of n paraffin in diesel matches melting point of flow improver, the function of flow improver is good. Hereby the functional mechanism of diesel flow improver is analyzed.