Abstract: The author holds that diachronism has a definite meaning. Theoretically, diachronism is prevalent, but in practical work it is not easy or workable anywhere to enumerate reliable data to prove its existence. At present some people determine diachronism rashly or prove it improperly, which leads to confusion in expression and description of modern stratigraphy to some extent and dilution of the importance of teconcept of diae ronism.