Palaeoflood information is based on the recognition and correct judgement to its carrier-slackwater deposits of palaeoflood. The basic way is grain-size analysis. After the working on the Yangtze River,Yellow River, Haihe River and Huaihe River for palaeoflood, a mark system of characteristics of grain size for palaeoflood slackwater deposits is first time given in this paper, which has wide use in palaeoflood study.