[Objective] The objective of this study is to study the relationships between straw decomposition period and water absorption capacity of rice,wheat and rape straw in order to provide the theoretical basis for effective utilization of straw resources and field water management.[Methods] The method of nylon net bag under moisture saturation incubation was used.[Results] The decomposition rate of three crop straws was much faster at the early period than the following periods. The cumulative decomposing rates were 67.8%, 55.5% and 49.2% for rice, wheat and rape straw after 110 days, respectively. The light microscopy and infrared radiation(IR) analysis indicated that the composition, chemical structure and morphology of rice straws changed greatly compared to control treatment after 110 days incubation. The results of water absorption capacity showed that the saturated water of dried rice, wheat and rape straw were 3.87, 2.51 and 3.61 g/g, respectively. After 15 days incubation, the saturation water absorption of rice straw reached to the largest value of 5.17 g/g, then gradually decreased and tended to be stable. While the saturated water absorption of wheat and rape straw reached to the lowest, which were 1.87 and 2.59 g/g, respectively, after a 5 days incubation, and then increased gradually. The results of water absorption effects showed that the crop straw had the largest water retention capacity in the early decomposition period, and the water retention capacity decreased with the extension of the decomposing process. In brief, the water retention capacity of crop straw was influenced by both decomposition period and straw quality.[Conclusion] In the implement of straw returning to filed, especially for straw incorporation, we should pay more attention to straw water content, returning period and field moisture, to reduce the negative effect caused by straw water absorption.