As the thickness of the friction stir welding plate increases, the bearing capacity of the equipment cannot meet the requirement of high forging force of unilateral friction stir welding, so it is difficult to form a good quality weld. To reduce the forging force on the equipment, a dual-robot mirror friction stir welding process is proposed in this paper. The variation and influencing factors of the forging force are studied, and the feasibility and superiority of the process are verified. Firstly, a thermodynamic coupling finite element simulation model is established to obtain the variation law of the forging force with plate thickness, and the conclusion that the dual-robot mirror friction stir welding process can reduce the forging force is obtained. After that, the forging force estimation model is established, and the experimental correction and verification of the model based on data-driven is carried out to realize sensorless forging force monitoring. Finally, the mirror welding experiment, three-group process parameter multi-factor experiment, and unilateral friction stir welding comparison experiment are carried out on the prototype platform. The variation of the forging force with time and process parameters is obtained, and it is proved that the dual-robot mirror friction stir welding process can reduce the forging force on the stirring head and the bearing capacity requirement of the equipment.