The invention relates to an energy-saving system for a reinforced concrete outer-casting built-in heat insulation wall body structure and a construction method thereof, which can effectively solve the problems of external wall heat insulation of a high-rise building with high heat insulation requirement and cracking and falling of a heat insulation layer. The system is produced by a wall body and reinforcing steel bars in the wall body which are cast by concrete. The method comprises the following steps that: a reinforcing steel bar frame of the wall body is produced and arranged, namely parallel vertical reinforcing steel bars and indoor floor-connecting board tie bars perpendicular to the parallel vertical reinforcing steel bars are bound together, a grid board is hoisted and externally suspended outside the vertical reinforcing steel bars of the wall body, and is bound with the vertical reinforcing steel bars of the wall body by hooked bars and binding steel bars as well as framework positioning bearing bars, then a framework is arranged on the framework positioning bearing bars and is firmly bound with reinforcing steel bars of beams and boards of a floor, lapping bars are reserved, a heat insulation grid board is firstly arranged, then reinforcing steel bars of corbel brackets of a balcony and an air conditioning board external wall are bound, finally the concrete is cast, the curing is performed, and the framework is removed to repair the structure. The system has good heat insulation effect, energy conservation, environmental protection, simple method, and huge economic benefit and social benefit.