Abstract: The voltage loss in the downhole cable for the electric submersible pumps in ultra-deep wells directly affects the starting performance of the motor. Excessive cable voltage drop will cause the motor terminal voltage significantly lower than the rated value, resulting in insufficient starting torque. Multi-field coupling simulation is the key to the accurate prediction of cable voltage loss and the study of voltage drop law. Based on the multi-physics field coupling of electro-thermal-flow, a simulation model for predicting the voltage loss in downhole cable for electric submersible pumps in ultra-deep wells is established. The synergistic mechanism of cable Joule heat, geothermal gradient, and tubing fluid heat transfer on voltage loss is systematically analyzed. The effects of current intensity, geothermal gradient, and cable structure parameters on voltage loss are explored, and optimization suggestions are given. The research results show that the voltage loss is strongly coupled with the cable temperature field. The contribution of Joule heat, tubing fluid heat transfer, and geothermal gradient to cable temperature rise accounts for 54.68%, 6.09%, and 39.23%, respectively. The voltage loss increases nonlinearly with depth. The current intensity has a significant effect on the voltage loss. The voltage loss can reach 32.48% when the working current is 140 A for the cable with a pump depth of 6, 000 m in ultra-deep wells, which will have a serious impact on the motor start-up. The established multi-field coupling simulation model realizes the simulation analysis of cable voltage loss and temperature field in ultra-deep well, addressing the challenge of prediction deviation caused by ignoring multi-field coupling effect in traditional methods, and provides strong technical support for the design and selection of downhole cable for electric submersible pumps.