With the development of science and technology, the application to modern warfare of weapon systems identified with modules of laser ranging, infrared imaging and low-light viewing are more and more frequently nowadays. Their combat accuracy depends much on the parallelism of gun line and laser axis. In consequence, it is of great significance to detect and correct the non-parallelism for improving the aim and combat accuracy. The axis parallelism detection exists in not only the production and assembly, but also the adjustment in the combat clearance or field environment. It is the basic to ensure these multi-axis systems work efficiently, and is an essential link of system installation, use and maintenance. There are laboratory method and field detection method to detect the non-parallelism of gun line and laser axis. Now the laboratory method is more mature. At the same time, as it is difficult to avoid vibrations, temperature differences and harsh environment in the field detection, the axis parallelism detection in field environment is an important research direction. One of commonly used methodsaxis parallelism detection needs complex operation, specific environment, and using target, and could not realize the axis parallelism on-line detection in the fighting or running environment. To solve problems above, a multi-optic axis detecting system using a novel image processing way without target is presented in this paper. By using the method of digital image processing, it can test the non-parallelism of gun line and laser axis accurately, as the standard and correction parameter of the weapon systems axis parallelism. The system has advantages in small size, portable type, high precision, and no target. It is suitable for field detection. This paper introduces the operating principle of the system of axis parallelism detection based on digital Image without target, and focuses on the principle, realization method and experimental results of the parallelism detection of gun axis and the system of axis parallelism detection based on digital image without target. Corresponding author: Xia Wang, Associate Professor, Research interests in terms of photoelectric detection, infrared imaging system etc., TEL: 13811670583; Email: angelniuniu@bit.edu.cn