Applying biodegradable chelating agents to assist in phytoremediation is a promising method to increase the remove efficiency of metal pollutants from contaminated soils. The effects of biodegradable chelating agents on improving the phytoremediation capacity in uranium (U) and cadmium (Cd) contaminated soil was investigated using sunflowers, which were grown in pots containing soil with U and Cd added at 15 mg/kg. After 2 months of growth, citric acid (CA), oxalic acid (OA) and ethylenediamine disuccinate (EDDS) at various concentrations (0, 2.5, 5.0 and 7.5 mmol/kg) were applied. The results showed that plant biomass decreased by 12.12% for shoot and 15.74% for root under U and Cd combined stress. Meanwhile, chelating agent treatments, especially with EDDS, enhanced U and Cd stress in plants by decreasing biomass, inhibiting photosynthesis, and increasing malondialdehyde and H2O2 levels. The U uptake of plants after CA addition was significantly greater than that after OA and EDDS addition. Nevertheless, EDDS addition has better effects on Cd uptake than CA and OA addition. U and Cd remove efficiencies reached the maximum following the application of 5.0 mmol/kg CA and 5.0 mmol/kg EDDS, which were 177.48% and 181.51% higher than that of the control, respectively. Furthermore, the bioavailable U content in soils treated with CA were higher than that in soils treated with EDDS, whereas bioavailable Cd content significantly increased due to EDDS addition. These results suggest that biodegradable chelating agents have significant effects on improving the U and Cd phytoremediation potential of sunflowers. •EDDS enhanced the adverse effects of U and Cd in sunflower.•CA and EDDS effectively increased the uptake and translocation of U and Cd.•CA significantly promoted U bioavailability in soil-plant system. EDDS has better effects on mobilizing Cd in the soil.•CA and EDDS improved the phytoremediation efficiency of sunflower.