The research contents of this paper include: according to different application requirements, three different LIBS online detection system solutions are proposed. Design optimization of lens parameters of optical system using zemax according to different LIBS system requirements. Test, optimize, and calibrate different LIBS online detection systems. Three LIBS systems were used for on-line detection of different molten metal compositions. The specific research content and related innovative research results are as follows: (1) Aiming at the problem of low efficiency of long-distance laser focusing and spectral collection, the methods of long-distance double-pulse laser focusing and Cassegrain telescope collection are proposed. In the intensity of the excited plasma, the double pulse laser can be enhanced several times compared to the single pulse laser. and the laser beam expanding focusing scheme is adopted to reduce the size of the focused spot and increase the laser power density. The intensity of plasma excited by laser at long distance is guaranteed. The structure of Cassegrain telescope is used to collect spectra, which ensures the effective transmission of spectral signals over long distances. Aiming at the influence of harsh environment such as liquid slag and smoke in metallurgical field, the structure of immersion and argon blowing with probe is proposed to avoid slag and building a stable plasma excitation environment. (2) Design an autofocus LIBS system with a shared Cassegrain telescope structure. Improvements and simplifications based on the ChemCam system, taking into account the cost and consistency issues that are of greater concern to industrial applications. Using double pulse structure to enhance plasma signal, and the main mirror is designed with only one variable of curvature radius and no Schmidt plate, which greatly reduces the processing cost and energy loss. The dichroic mirror is placed outside the Cassegrain telescope at an angle of 45 degrees, without considering the influence of light blocking, which reduces the difficulty of assembly and debugging, and makes it easier to ensure the consistency of mass production. Automatic focusing at different measuring distances is realized. The LIBS analysis technology was applied to on-line detection of rare earth elements in molten magnesium alloys for the first time. (3) For the application needs of the steel industry, the sub-mirror shielding part of the Cassegrain telescope is skillfully utilized to reflect the laser focusing light path to the spectral collection optical path. An optical structure is realized, in which the focused optical path and the collected optical path are coaxial. The problem that the confocal LIBS system can not detect the deep ultraviolet band is solved, and the laser energy loss is reduced and the spectrum collection avoids the influence of chromatic aberration. For the first time, the remote double pulse LIBS technology was successfully applied to the on-line detection of molten steel components in 40-ton ladle. (4) Facing the requirement of miniaturization and engineering system for online application in aluminium processing industry, the remote LIBS system is further simplified and optimized, a single-pulse laser, spectral collection system uses a lens set instead of the Cassegrain telescope and the optical structure of focusing and collecting optical paths are coaxial. and a compact LIBS system is proposed to realize online application in aluminium processing industry.