The invention provides a precast side wall splicing construction method for an open-cut assembled subway station. By adoption of the precast side wall splicing construction method, the demand on the labor force and the consumption of on-site building turnovers of traditional subway station cast-in-place side wall construction are greatly lowered, wet-working operation on the construction site is greatly reduced, such pollution as dust raising, noise and the like of the construction site are relieved, and the influence on the ambient environment of the construction site is reduced. On the construction site, a precast side wall splicing trolley is adopted to guarantee accurate aligning and splicing of side walls, so that the mechanical degree of construction is improved, and the project demand on the labor force is lowered. Owing to use of lots of precast members, the overall construction quality and the construction efficiency of related projects are remarkably improved, and a foundation is laid for industrial construction popularization.