The invention relates to a high arch dam spread foundation structure applicable to a low-variable-modulus foundation. The spread foundation structure aims to enlarge the thrust influence range of an arch dam, homogenize stress distribution of the arch dam and the foundation and reduce the stress values of the arch dam and the foundation by spreading the foundation structure so that the high arch dam can be built in the low-variable-modulus foundation on the premise that economical efficiency and reasonability are guaranteed and the technology is feasible. According to the technical scheme, the foundation structure is provided with the high arch dam built between a left bank and a right bank; a dam body of the arch dam is divided into a plurality of dam segments through arch dam transverse seams; low-variable-modulus foundation rock masses are distributed on parts of foundation segments of the dam body of the arch dam through exploration; a spread foundation concrete structure is distributed within the range where the low-variable-modulus foundation rock masses are distributed; the spread foundation concrete structure and the dam body of the arch dam are poured as a whole; besides, the arch dam transverse seams extend to the bottom of the spread foundation concrete structure. The high arch dam spread foundation structure is applicable to building the high arch dam in the low-variable-modulus foundation.