The invention provides a large-heat input gas-electric vertical welding gas shielded flux-cored wire based on a grain refinement mechanism. The large-heat input gas-electric vertical welding gas shielded flux-cored wire comprises a flux-cored wire outer skin and inner powder, wherein a low-carbon steel cold-rolled steel strip is adopted by the flux-cored wire outer skin, and the flux-cored wire comprises, by mass, less than 0.06% of C, less than 0.3% of Si, less than 0.8% of Mn, less than 0.02% of P, less than 0.01% of S, and the balance Fe and inevitable impurities; the powder comprises, by mass, 1-5% of one or two of titanium dioxide or rutile, 2-6% of quartz sand, 3-8% of feldspar, 1-5% of magnetite, 2-5% of one or more of lithium oxide, sodium oxide or potassium oxide, 10-15% of low-carbon ferromanganese, 3-8% of aluminum powder, 3-8% of ferrotitanium, 2-6% of nickel powder, 2-6% of rare earth silicon, 1-3% of magnesium powder, and the balance reduced iron powder; and the powder accounts for 15-20% of the total mass of the flux-cored wire. The flux-cored wire can be used for 100kj/cm-300kj/cm large-heat input gas-electric vertical welding of a thick steel plate.