Abstract: Abstract: Lodging is one of the important constraints to high quality and stable production of rice. This study was conducted in the Key Laboratory of Efficient Irrigation-Drainage and Agricultural Soil Water Environment in Southern China, Ministry of Education, Nanjing, China from May to October in 2013 to investigate the response of alternate flooding and drought stress on biomass production and distribution in different organs of rice (Oryza sativa L. Nanjing 44) and their relationships with morphological and mechanical traits as well as chemical concentration relating to stem lodging characteristics under alternate drought and flooding stress during different growth stages. Five treatments, alternate stress of light drought-flooding-light drought in tillering stage (T-LD), alternate stress of heavy drought-flooding-heavy drought in tillering stage (T-HD), alternate stress of light drought-flooding-light drought in jointing stage (J-LD), alternate stress heavy drought-flooding-heavy drought in jointing stage (J-HD) as well as irrigation with shallow water depth (0-5 cm) for all the stages except for yellow maturity (CK), were set up. In this experiment, changes of chlorophyll contents and photosynthesis rates of flag leaves after the flowering stage as well as the percentage of dry-matter exportation from stem-sheath (PDESS) and the percentage of the dry-matter transformation from the stem-sheath part to the grain part (PDTSS) were investigated. In additon, the stem morphological and mechanical traits as well as chemical concentrations and their relationships with stem lodging characteristic were studied. The results showed that there were significant quadratic curve relationships between chlorophyll contents and days after transplanting (P<0.01). In addition, there were significant positive linear relationships between chlorophyll contents and photosynthesis rate of flag leaves after the flowering stage (P<0.001). Compared with the CK, T-LD and T-HD treatments significantly increased the leaf areas, chlorophyll contents and photosynthesis rates of flag leaves after the flowering stage, but significantly decreased the PDESS and the PDTSS; the leaf areas, chlorophyll contents and photosynthesis rates of flag leaves after the flowering stage of alternate stress of drought-flooding-drought in the tillering stage were about 1.1-1.2 times as those of CK, respectively; the PDESS and PDTSS were 32% and 22% of those of CK, respectively. Compared with the CK, J-LD and J-HD treatments significantly increased the chlorophyll contents and photosynthesis rates of flag leaves after the flowering stage but did not significantly increase the leaf areas, the PDESS and the PDTSS. The leaf areas of flag leaves, the PDESS and the PDTSS of alternate stress of drought-flooding- drought in the jointing stage were 84%, 33% and 37% of those of CK, respectively; the photosynthesis rates of flag leaves after the flowering stage were 1.19 times as those of CK, respectively. Compared with the CK, J-LD and J-HD treatments significantly increased the safety factor against stem breakage, which might be related to the improved dry-matter translocation amount and translocation efficiency from stem-sheath to grain part of rice plants as well as the increased gravity centre height and the ratio of gravity centre caused by the improved translocation amount of carbohydrate generated in flag leaves to the grains. There were significant negative relationships between harvest index and safety factor. This study could provide valuable information for keeping high grain yield as well as improving lodging resistance of rice plant by irrigaiton methods.