The invention discloses a lead alloy slag reducing agent and application thereof in preparation of lead-acid battery grids. The lead alloy slag reducing agent is prepared from the following components: cork powder, wood carbon, sodium hydroxide and coal slag, wherein the mass ratio of the cork powder to the wood carbon to the sodium hydroxide to the coal slag is equal to (10-12) to 10 to 1 to 20.A preparation method of the lead-acid battery grids comprises the following steps: (1) sequentially adding particulate lead, calcium-aluminum alloy and tin ingots into a lead smelting furnace according to a grid alloy formula, and enabling all the raw materials to be melted to obtain liquid lead; (2) controlling the temperature of the liquid lead to 510-560 DEG C, then adding the lead alloy slag reducing agent into the liquid lead, stirring, and then removing dross; (3) casting the lead-acid battery grids by using the liquid lead without the dross. The lead alloy slag reducing agent provided by the invention is simple to prepare and low in cost, and can greatly reduce the generation of lead slag, reduce the loss of alloying elements, improve the consistency between lead and other alloy components and improve the casting quality and casting performance of the grids.