The invention relates to an assembly technique for split crankshafts of a small aero-engine. The assembly technique comprises the five steps of, assembling bearings and a bearing seat, processing andpairing the plurality of crankshafts with the same structure, installing butt joint sleeve rings into a bearing seat assembly, heating the assembly along with an external positioning tray tool, butting the crankshafts and fixing the bearings and the bearing seat; the problem that when a conventional bearing is pressed into a bearing seat through cold pressing, a certain amount of plasticity deformation of a bearing seat hole of a light alloy material is generated so that the actual interference amount can be insufficient and the reliability can be affected is solved, and the installation position of the bearing is accurately in place is also ensured; the problems of scratch or safety and environmental protection caused by conventional methods of crankshaft butt joint are solved; and accurate in-place and qualified rate of butt assembly of the split crankshafts are guaranteed, and reliability of products is effectively improved.