In the investigation,first,an automatic control system of narrow-gap magnetic controlled tungsten inert gas ( TIG) arc welding for thick plates is designed,and the principle of avoiding the defect of incomplete penetration on side walls by adopting an alternatively-changed magnetic field to deflect welding arc towards left or right periodically is introduced.Next,by taking a programmable logic controller ( PLC) as the core and based on its characteristic,a control system is overall designed,and two independent arc length control systems are designed to keep the tungsten electrode exactly in the middle of the gap and at a relatively constant height.Then,a sensing and motion control system is designed and established,and effective protection of weld seam is realized by employing high-precision gas sensors to detect and control the actual flow rate of shielding gas and by implementing the realtime measurment and control of the distance between the shielding covering and the substrate.Finally,a suitable welding power source is selected and modified to realize a digital control,and a vision monitoring system for industry is set up.The results of welding experiments on thick titanium plates demonstrate that the designed automatic control system is reliable and stable,and it has the potentiality to be promoted in industry.