According to the study on flood hydrograph, the new concept of specific flood hydrograph and its connotation were proposed.Meanwhile, the external and essential implication of engineering flood prevention standard were expounded. And then the re -lationship that was the new concept of external frequency of flood hydrograph between specific flood hydrograph and flood preven -tion standard was drawn. The specific flood hydrograph of three types of transformation and its calculation method were elaborated.The advantages and disadvantages of the common multiple method and the common frequency method of specific flood hydrographwere pointed out. A brief introduction about the basic idea of new calculation method of specific flood hydrograph was given.