A construction method of an open excavation prefabricated underground structure includes the steps: 1, leveling a ground to serve as a foundation pit supporting structure and a base cushion layer; 2, mounting a fixing support at an assembly initial end; 3, hoisting prefabricated components to assemble a structural base plate; 4, assembling a special assembly trolley on the structural base plate; 5, assembling side walls and top plates of the prefabricated components; 6, backfilling upper portions of the top plates and outer portions of structural side walls to complete assembly of the underground structure. The construction method has the advantages that construction mechanical level is improved; operating personnel amount is greatly decreased, operating efficiency is improved, engineering construction quality is improved, resource consumption is reduced, environmental influences are lowered, and construction period can be greatly shortened; amount of mould plates in cast-in-situ concrete structures, peripheral shields, scaffolds and wastes is greatly decreased to relieve field operation intensity and reduce construction input; construction in chilly winter is realized, engineering construction quality is improved, and potential safety hazards such as fire hazards and the like in construction of traditional cast-in-situ concrete structures in winter can be avoided.