Abstract: With the development of nuclear power, marine engineering, pressure vessels, and high-end energy equipment toward larger size, thicker sections, and higher reliability, conventional multi-pass arc welding processes are increasingly unable to meet engineering requirements in terms of welding efficiency, heat input control, deformation suppression, and quality consistency. Laser welding offers advantages such as high energy density, low heat input, and ease of automation. However, under atmospheric high-power welding conditions, laser welding is susceptible to shielding effects caused by metal vapor and plasma plumes, which reduce energy coupling efficiency, destabilize the keyhole and molten pool, and induce defects such as porosity, spatter, and weld collapse. Low-vacuum laser welding establishes a sub-atmospheric environment in the welding region, reducing the density of the gaseous medium and suppressing the absorption and scattering of the laser beam by the plasma plume, thereby improving penetration capability and welding stability. This paper systematically reviews the technical principles of low-vacuum laser welding, domestic and international research progress in welding processes, and the development status of vacuum and local-vacuum welding equipment. Typical application scenarios, including nuclear power equipment, aerospace structures, deep-sea equipment, pressure vessels, and offshore wind power structures, are also discussed to analyze the engineering application prospects and key challenges of this technology. The results indicate that low-vacuum laser welding has significant advantages in achieving efficient, high-quality, and low-deformation joining of thick-section structures. The miniaturization, dynamic sealing, stable pressure control, and intelligent control of local low-vacuum equipment will be important directions for future engineering applications.