As a sensitive biological indicator,earthworms are widely used to monitor various pollutants of soil and provide an early warning for soil pollution.However,because many indices are involved in the exposure-induced oxidative stress response,practical applications of these indices are quite inconvenient.Therefore,it is appropriate to investigate the key monitoring index for use in early warning and pollution monitoring.Using Eiseniafetida as an experimental model in an indoor simulation experiment,the mathematical modelling of the effect on oxidative stress in earthworms under cadmium (Cd) stress was studied.The test lasted 40 d,with the removal of one earthworm every 10 d.The Cd2+ concentration gradient was set as 0,1,10,20,100,200,400,and 800 mg kg-1 dry weight.The earthworms were divided into two sections from the clitellum for the determination of total protein (TP) and peroxidase (POD),superoxide dismutase (SOD),glutathione-S-transferase (GST),glutathione peroxidase (GPX),catalase (CAT),malondialdehyde (MDA),and acetylcholinesterase (ACHE) activities.Results showed that POD was the key index of oxidative stress in head tissues after 10 d of exposure,TP was the key index at 20 d,and POD became the key index again at 30 and 40 d.By contrast,in tail tissues,MDA and SOD were the key indices at an exposure time of 10 d,GPX at 20 d,CAT and TP at 30 d,and POD and MDA at 40 d.These results contribute to establishing a scientific method for ecotoxicological diagnosis and revealing the mechanism of soil Cd toxicity.