Internal fixation with steel plates is a common method for the therapy of lower limb fractures. Due to the disadvantages of traditional surgical methods such as high trauma and complications, the application of robots for minimally invasive surgery has become a popular research direction. In this article, we have designed and simulated the overall structure of a steel plate internal fixation device used in conjunction with a robotic arm. Firstly, a finite element model of cortical bone drilling was established based on ABAQUS to obtain the torque and axial force when drilling the cortical bone. Then a virtual prototype of the robot-assisted lower limb bone fracture plate internal fixation device was designed, and the outer shell, drilling mechanism, nail placement mechanism, nail supply mechanism, drive mechanism, and guide sleeve were refined based on the overall mechanical structure of the device, so that the device could continuously perform drilling and nail placement. Afterward, the device was simulated based on ADAMS, and the motion curves of each component were obtained to verify the feasibility of the device’s working principle and to validate the performance of the servo. Finally, the modal analysis of the device was carried out with the finite element software ABAQUS, and the modal parameters of the first six orders were obtained, which were compared with the operating frequencies of the motor and the servo to verify that the device is not easy to resonate during normal operation, and the static strength checks of the key components were carried out, and the stress and deformation clouds and upper limits of flexural values of the components were obtained, which proved that the structure has stability.