The invention relates to a post-arranged inner supporting structure for partitioned excavation of a super-large deep foundation pit and a construction method. The method is characterized in that an outer support pile, an inner support pile and a pile top solidified soil layer are arranged on the outer side wall of the foundation pit firstly, and lower top beam positioning bar bodies and upper top beam positioning bar bodies are implanted in the inner support pile; then step-slope excavation is conducted on soil in the middle of the foundation pit from the middle to the two sides of the foundation pit, and a pit bottom drainage layer, a geogrid layer, a post-cast strip ground beam, anti-floating anchors and a solidified cement soil layer are arranged in the positions, at the bottom of the foundation pit, of the excavated area; and a post-arranged inner support is arranged between an adjustable bracket on the post-cast strip ground beam, and a pile top beam, then soil below the post-arranged inner support is excavated, pit bottom reinforcing piles, a pit bottom drainage layer, pit bottom drainage ditches and a solidified cement soil layer are arranged at the pit bottom below the post-arranged inner support, and finally, the post-arranged inner support is removed. The force-bearing behavior of a foundation pit supporting system can be improved, in addition, the excavation rate of the foundation pit can be increased, and the amount of materials for foundation pit supporting can be decreased.