The invention discloses a method for welding a bottom plate of a large flat-bottom storage tank, which has the advantage of simple processing process and can effectively reduce the welding deformation of the bottom plate of the large flat-bottom storage tank, improve the evenness of the bottom plate and prolong the service life of the storage tank, solving the technical problems of large bottom plate deformation, poor storage tank strength and rigidity, service life reduction, and the like in the prior art. The method mainly comprises the following steps of: butt jointing or lap jointing a plurality of steel plates to form a bottom plate of a round flat-bottom storage tank; then paving a plurality of fan-shaped steel plates around the circumference of the bottom plate of the flat-bottom storage tank to form an annular edge plate; partially welding adjacent fan-shaped steel plates from outside to inside; then coaxially putting cylindrical wall plates; and welding according to a weldingsequence of spot fixing and welding from outside to inside.