In the assembly process of some products,whether the size of assembly gap meets the demand directly reflects the level of assembly quality.Traditional small gap measurement often adopts manual measurement,the accuracy and consistency of measurement is difficult to ensure.The research and application of digital on-line measurement technology is imminent.Digital measurement technology is greatly affected by external factors such as environment and illumination.Aiming at the problem of automatic measurement of object end clearance in the assembly process,a visual measurement system was proposed that could adapt to the structure of the object,which could realize the on-line measurement of the small gap of the assembly.The principle of the measurement system was analyzed,and the design of the system was explained.The application effect of the system in practice was tested and verified.The results show that the designed system meets the requirements.