Rice-crab culture system is a new ecological complex that combines rice culture with that of Eriocheir sinensis. The growth of Spirogyra will seriously affect the growth of rice and rice yield. To determine the effects of rice-crab culture systems on Spirogyra in paddy fields, periodic sampling was conducted in rice-crab culture systems with different megalopa densities (60 ind/m², T₁; 90 ind/m², T₂; 90 ind/m², T₃) and in conventional paddy field (CK) at five key dates (June 17th, June 28th, July 17th, July 30th and August 15th). The results show that the average dry weight of Spirogyra of the rice-crab culture fields and the conventional paddy fields were significantly different in the whole experiment period, and this shows the difference crab stocking density had obvious control action on the biomass of Spirogyra in the paddy fields. A certain amount of Spirogyra appeared on June 17, especially the average dry weight of Spirogyra of conventional paddy field (CK) and rice-crab culture field with low density of megalopa (T₁) were obviously higher than that of the other two treatments (P<0.05), and the average dry weight of Spirogyra of rice-crab culture field with high density of megalopa (T₃) was significantly lower than other treatments (P<0.01); On July 17th, the CK maximum biomass of Spirogyra, an average of dry weight 46.91±5.35 g, and area covered in 65%-75%, and other treatments also reached a maximum of the whole experiment period, and significant difference of different treatments(T₃ < T₂ < T₁ < CK, P<0.05; T₃ 1 < CK, T₂ < T₁ < CK,P<0.01); Sampling results on August 15, that there was no moss in rice-crab culture field with high density of megalopa (T₃), the T₂ lowest biomass of Spirogyra, the highest amount of in the CK, there were significant differences in different treatments (T₃ < CK, T₂ < CK, T₁ < CK, P<0.01; T₂ < T₁, P<0.05).The study provided additional theoretical guidance about biological control of Spirogyra and sustainable development of rice-crab culture systems. It provided another means of promoting the development of rice-crab culture technology.