The invention discloses a laser welding method for narrow space of a casing. The laser welding method comprises the following steps that oil removal treatment is carried out on a section of a joint ofa part and two sides of the joint before welding; a support plate, an outer casing and an inner ring front section are assembled, the inner ring front section and the fixing support plate are positioned, and the matching clearance is ensured to be less than or equal to 0.1mm; a robot optical fiber laser welding device is adopted, and inert gas protection is carried out on the front sides of weldjoints; corresponding welding parameters are controlled, and laser welding is carried out; and after welding is completed, the weld joints of the support plate and the inner ring front section are checked, the appearance of the weld joints is ensured to be in smooth transition with base material, the surfaces of the weld joints are free of cracks, burnthrough, overlaps and welding spatters, an x-ray bar anode coloring inspection is carried out, and no welding defect is found. By the adoption of the laser welding method, the welding heat input quantity and the weld joint forming quality are effectively controlled, welding deformation is reduced, the welding quality and the stability are greatly improved, finally, the problems of welding deformation of the gas inlet casing and testing and repairing cracks are solved.