The effects of applying four kinds of decomposition agents (DA1, DA2, DA3 and DA4) on straw decomposition and water environmental risk in paddy field under the conditions of returning wheat straw to the field were studied. The results showed that the decomposition rate of wheat straw was increased by different decomposition agents in the first four weeks, and the first two weeks was the rapid decomposing period. In addition, DA3 and DA4 had better effect on promoting the decomposition of wheat straw. The decomposition rate of wheat straw of DA3 and DA4 were 36.5%~37.4% in the first two weeks, which was 5.1~6.0 percentage points higher than that of CFS treatment (31.4%). Compared with the CFS treatment, the four treatments increased the COD (chemical oxygen demand), nitrogen and phosphorus contents of field surface water within 3 weeks after wheat straw returning to the field, which increased the water environment risk in the early stage of rice growth. Among them, DA2, DA3 and DA4 treatments had relatively high risk of surface water environment within 3 weeks after returning wheat straw to the field, while there was no significant difference between DA1 and CFS treatments. It can be seen that the application of decomposition agent can accelerate the decomposition of wheat straw and increase the water environment risk in the early stage of rice growth. Therefore, it is recommended to apply decomposing agent carefully in the rice season in the southern water network area.