Abstract: This paper introduces a new auto-body spot weld quality monitoring system using electrode displacement.This system solves the interference problem of sensor mounting by designing special fixture,and can be successfully applied on the portable welding machine.It has capable of making diagnosis of process variations such as surface asperities,shunting,worn electrode and splash with real-time electrode displacement.Through confirmation of application in the workshop,monitoring system has good stability and reliability,and is suited for monitoring welding quality in production.