The invention relates to the field of building construction, and provides pebble stratum deep and large foundation pit underground diaphragm wall retaining wall slurry. The pebble stratum deep and large foundation pit underground diaphragm wall retaining wall slurry is prepared from, by weight, 1,000 parts of water, 75-85 parts of bentonite, 3-5 parts of sodium carbonate and 0.5-1.5 parts of sodium polyacrylate. The invention further provides a preparation method of the pebble stratum deep and large foundation pit underground diaphragm wall retaining wall slurry. The pebble stratum deep and large foundation pit underground diaphragm wall retaining wall slurry has the advantages that sodium polyacrylate is adopted for replacing traditional CMC, the slurry wall protecting performance and theslurry recycling rate are greatly increased, and meanwhile corresponding environment contamination and material consumption are reduced; meanwhile, various indexes of the slurry prepared through theformula can meet the requirement of circulating slurry indexes, the wall protecting effect is good, and the groove wall stability is good.