For the requirements of contact force in the process of industrial robots such as polishing, a fuzzy PID constant force control method based on particle swarm (PSO) optimization is proposed. Firstly, the force and gas flow model of the flexible force-controlled flange are analyzed to establish the system model of the flexible force-controlled flange device; secondly, the fuzzy PID controller based on PSO algorithm is designed so that the control parameters of fuzzy PID can be adaptively adjusted, and the fuzzy PID control optimized based on PSO algorithm is compared with ordinary PID, fuzzy PID and PSO algorithm through MATLAB simulation; finally, the experimental verification of the constant force control output performance of the flexible force-controlled flange is carried out by building a LabVIEW-based grinding experiment platform. The simulation experiment results show that: Compared with the traditional PID and fuzzy PID control methods, the fuzzy PID control optimized based on PSO algorithm has no overshoot, the system response is faster, and the system reaches stability at 0.43 s; the actual contact force output error of the flexible force control flange is less than 0.85 N when the constant force grinding experiment is conducted; the roughness of each area of the shell surface after grinding is stable between Ra0.1~Ra0.2. The method can effectively suppress the contact pressure fluctuation and has stronger robust performance.