The invention discloses an energy-saving foundation of a lightweight steel structure fabricated type building. The energy-saving foundation of the lightweight steel structure fabricated type buildingcomprises a foundation base and a floor system arranged on the foundation base, wherein the foundation base comprises a foundation wall, multiple cavities enclosed by the foundation wall, a joist foundation and a heat insulation board; the foundation wall further comprises an outer foundation wall, an inner foundation wall and a first bearing beam arranged on the top surfaces of the outer foundation wall and the inner foundation wall; the floor system comprises side beams, floor beams and veneers paved on the side beams and the floor beams; the side beams further comprise inner-side beams, outer-side beams and connecting pieces arranged between the inner-side beams and the outer-side beams; and a stiffening piece is arranged in an opening of a butt-joint corner of the two inner-side beams.According to the energy-saving foundation provided by the invention, the heat-insulation and heat-radiation effects of a house can be improved, and the energy consumption of the house is reduced; andthrough the cooperation of double-side beam stiffening structures, side beam corner stiffening structures and a joist support, the structure strength of the floor system part can be improved in multiple ways, so that the bearing capacity is improved through the cooperation of the floor system and the foundation base.