Twin-arc integrated cold wire hybrid welding system has been established. Effect mechanism of cold wire position on welding process was studied, which included effects of cold wire position on its heating and melting situation and weld surface formation. The results showed that the melting of cold wire just depended on front end of weld pool, and melting effect of weld pool on cold wire was not sufficient when cold wire was fed in front of two leading wires. The end of cold wire would contract the bottom surface of weld pool with continuous feeding of cold wire, and the droplet melted at the wire end was ejected and fell on base metal surface to generate globular spatter with moving backward of base metal. The temperature distributed on side of weld pool decreased with feeding cold wire in side of two leading wires. In this way, the molten metal flowed in weld pool would be affected, which led to final uneven weld formation. The cold wire was stably inserted into weld pool when cold wires was fed behind two leading wires, and this was optimum cold wire position. Besides, the deposition rates increased with increase of feeding speed of cold wire and made little difference under two pulse phase differences (in-phase and reverse-phase pulse differences). The effect of arc heating and melting on cold wire was most intense at in-phase pulse current, followed by reverse-phase pulse current, subsequently direct current.