Abstract: In order to adapt the development of the resistance welding technology and improve the automation of the resistance welding machine, an automatic thickness measurement device with the photoelectric coder and the PLC high-speed counter was studied based on the structure and application situation of the resistance welding machine.The type, construction, working principle of the photoelectric coder were especially introduced.And actual application in this thickness measuring device was expounded.This device can feed the measured data back to the controller of the welding machine automatically, then the best welding parameters correspond with the thickness of the weldment were chosed in advance in order that the welding machine can weld the work piece automatically in sequence.The experimental results indicate that this thickness measurement device can achieve the precision demand of choosing the welding parameters and can satisfy the actual production well.