The invention relates to the technical field of reverse building method construction, in particular to a reverse building method construction gap reinforcement and template construction method. A reverse building process is adopted, and is a construction method of using a main body engineering underground structure as a foundation pit supporting structure and adopting a top-to-bottom design of theunderground structure. When soil is excavated to reach a designed elevation, the main body structure is started to build; and then, the downward excavation is continued until the designed elevation is reached. A reverse building method bottom mold implementation mode comprises bottom excavation-reinforcement connection-gravel backfilling-groove gap outer mold building. The method solves the problem of difficult operation during downward lengthening of reverse building method reinforcements, solves the problem of instable supporting of reverse building method construction gap bottom templates,and improves the construction speed and safety.