In order to carry the testing instrument to the required position in the horizontal/ inclined oil well, the downhole robot is designed. The downhole robot has a self driving force which is produced by the electrical motor in the robot, so it can carry the testing instrument reliably. In addition, the mechanical structure is simplified compared with that of the other downhole robot. Firstly, the mechanical structure design of the robot is introduced. The robot includes three parts: driving module, centralizing module, and base. Then, the driving force required and the power for the electrical motor in the driving module are calculated when the robot runs in the oil well. The simulation result shows that the downhole robot can work reliably in the oil well.