To determine the effects of rice-crab culture systems on water environment and rice yield in paddy fields, periodic sampling was conducted in rice-crab culture systems with different crab densities (18 ind/m², low density (LD); 54 ind/m², high density (HD)) and non-crab paddy field (CK) at six key growth stages (seeding stage, reviving stage, tillering stage, elongation stage, flowering stage, filling stage). The samples were analyzed for water quality and rice yield. The results revealed that the dissolved oxygen concentration of CK was higher than that in stocking crab paddy field in the rice-growing cycle. It was significantly different(P<0.05) between CK and stocking crab paddy field, when the rice was only in flowering stage. There was no significant difference (P>0.05) between LD and HD. At tillering stage, the ammonia nitrogen concentration of HD was significantly higher than CK (P<0.05). At filling stage, the total phosphorus (TP) concentration of CK was the highest, and it was significantly different (P<0.05) between CK and stocking crab paddy field. It was not significantly different (P>0.05) between LD and HD. It was lack of phosphate fertilizer from the beginning of tillering stage. It was lack of nitrogen fertilizer from the beginning of flowering stage and filling stage. It was appropriate to stock larval crab in paddy field, when rice was in tillering stage. Because concentration of nitrite and ammonia nitrogen decreased rapidly below the crab safe concentration. The rice yield of LD was significantly higher than that in CK and HD (P<0.05). The rice yield of CK was the lowest, and it was significantly different (P<0.05) between CK and HD.