Abstract: The model of UB (under body) in BIW (body-in-white) was built in the software Delmia. The welding paths which welding guns went through and the average distance between welding points were obtained through simulation by traditional method of welding path planning. The theoretical foundation of ACO (ant colony optimization) and its algorithm flow were illustrated. With the combination with welding characters of UB, the programming of ACO was designed according to the calculated ideology. As a result, the optimum solution of welding path in UB was achieved and the new parameters were also obtained by the simulation with the foundation of ACO. Compared with the former parameters, the results show that the feasibility of ACO applied in the planning of welding path was validated which would lay the foundation for the welding process in the future.