A mechanism spot welding system consists of three parts of the mechanical device, power supply devices, control devices. Spot welding in order to adapt the welding process requirements, the pressurizing mechanism (electrode holder) with two-stroke rapid pressure transmission through the switch stroke control handle to change the welding clamp opening degree can be divided into the wide open and a small opening to meet the welding operation requires. Usually state the short stroke of the welding tongs open when the control button to switch to the "power" position, pull the switch handle welding clamp tight pressure, while the current in the control system to complete a welding cycle to restore to the short stroke Zhang open state. 2 main power circuit of the power supply device by resistance welding transformers, thyristor unit, the main power switch, welding circuit components. At present, we use the Welding Electrode Manufacturers is the power of 200kVA secondary output voltage of 20V - 40V single-phase industrial frequency AC resistance welding machine. Line to produce a variety of models the electrode holder welding high strength steel and mild steel sheet, Welding Cable gun arm to pass a large mechanical force and welding current, welding clamp strength, stiffness, fever to meet certain requirements and should have good electrical and thermal conductivity, and electrode holder with water cooling, so I chose the welding electrode clamp arm can withstand 400kg pressure a new type of welding clamp. Control device control device to provide signals to control the resistance welding machine action on and off the welding current to control the welding current, fault monitoring and handling.