The invention relates to the technical field of constructional engineering, in particular to a semi-prefabricating construction process for a concrete thin-wall lower return eave. At present, two construction processes for the thin-wall lower return eave are provided, one process is in-place casting and the other one process is completely prefabricating the component. The cast-in-place lower return eave usually has such quality problems as cracking, crazing and the like; and when a completely prefabricated component is used, the upper part of the lower return eave and a main body structure need to be welded, welding seams are difficult to process, and easy to crack, so that the appearance effects are affected, and the quality is difficult to ensure. The semi-prefabricating construction process disclosed by the inveniton is adopted to split the lower return eave into structural units with simple structures and convenience in construction, so that prefabricating in advance is realized in site, the installation is completed during a construction process of the main work, and the joints are flexibly processed The semi-prefabricating construction process effectively solves the defects in the prior art, can improve the project quality, shortens the construction period, and saves the construction materials and expenses of labour. The semi-prefabricating construction process is particularly suitable for the construction of the thin-wall lower return eave which is 50mm-100mm in thickness.