The invention discloses a construction method of a large inclined rock surface foundation of a loose accumulation body of a steep slope. The construction method is characterized by comprising the following steps in sequence that firstly, the loose accumulation body in the range of a bridge foundation construction site is excavated and removed, and meanwhile, a buffer platform is arranged; the part, higher than the bearing platform bottom elevation and in the range of a bearing platform, of the large inclined rock surface is excavated to the bearing platform bottom elevation; the part, lower than the bearing platform bottom elevation and in the range of the bearing platform, of the large inclined rock surface is excavated into steps; a retaining wall is constructed to the bearing platform bottom elevation on the slope foot position of the loose accumulation body of the steep slope, backfilling is carried out on the excavated steps in a layered mode, and a pile foundation steel casing issimultaneously arranged in a buried mode; and filling is continuously carried out until the elevation of the buffer platform is reached to form a construction platform, a gantry crane and a drillingrig are installed, the bridge foundation construction is carried out, wherein the bridge foundation construction includes the construction of a pile foundation, after the pile foundation constructionis completed, excavation is carried out to the bearing platform bottom elevation, a pile head is removed, and finally construction of the bearing platform is carried out.