Abstract: Based on the data objective analysis, in this study, the optimal segmentation method (OSM) is used for quantitative division of the climatic normal series of pentad rainfall in the 610 meteorological stations over China. According to the climatic characteristics of pentad rainfall over China, three rainy-season division schemes are made for the yearly pentad rainfall series which would be simultaneously satisfied with the minimum variances in the same phase and the maximum differences among phases: one includes rainy and dry seasons; another one contains the relatively wet and relatively dry seasons in a rainy season, and the dry season. The third one consists of the spring rainy season, main rainy season, autumn rainy season and dry season, and is applied for the regions with single or three peeks in their pentad rainfall series. It demonstrates the objectivity and effectiveness of OSM for rainy season division in China; and the rainfall phases divided by OSM are rational and of meteorological interpretations.