The effects of long-term straw return to the field (CFS) on the changes of water environment risk indicators such as chemical oxygen demand (COD), nitrogen (N) and phosphorus (P) in surface water and rice yield were studied by 11-year positioning experiment. The results showed that compared with CF treatment (without returning straw to the field), CFS treatment reduced rice yield by 3.5%, but the difference was not significant, mainly because CFS treatment reduced the number of effective ears of rice. Compared with CF treatment, the content of total nitrogen (TN) and total phosphorus (TP) in surface water treated with CFS showed a trend of first decreasing and then increasing, while the ammonium nitrogen (NH₄⁺-N) /TN in the surface water at the initial stage of return to field was decreased, but the aquatic environment risk of COD in the surface water was increased by 69.4% on average. However, the COD of field surface water of CFS treatment was lower than the water quality requirement of paddy field crop irrigation in the national standard “Farmland Irrigation Water Quality Standard” (GB5084—2021) (COD is 150 mg/L). Therefore, the CFS treatment reduced the water environment risk of N and P in the surface water at the initial stage of straw return to the field, and increased the water environment risk of COD in surface water, but had no significant effect on rice yield.