The invention discloses a micro-plasma arc welding gun which comprises a tungsten electrode press head, a sealing ring, a tungsten electrode clamp, an upper gun body, an insulator, a lower gun body, a centering support pillar, a centering sleeve, a gun mouth, a protecting gas cap, a tungsten electrode and a handle. The micro-plasma arc welding gun disclosed by the invention has the following beneficial effects that the upper gun body and the lower gun body are high in heat conduction efficiency; due to an optimally designed lower gun body cooling water channel, the heat conduction travel is reduced, and the cooling effect of the gun mouth is improved; the tungsten electrode clamp, the centering support pillar and the centering sleeve can ensure that the tungsten electrode is coaxial with the gun mouth; on the premise that advantages such as small plasma diameter and high energy density are maintained for the gun mouth, the stiffness of the plasma arc can be reduced, low current overlaying under 50 amperes can be achieved, and the sizes of thin-wall components made of materials such as titanium alloy and nickel base alloy can be recovered through overlaying. In addition, the micro-plasma arc welding gun is compact in structure, so that the accessibility of the welding gun in working is improved.