The invention relates to a steelmaking dephosphorizing agent interface modifier and a preparation and use method thereof. The modifier comprises the following components in corresponding mass ratio: La2O2S, Zr, Cr, CaO and CaCl2. The preparation method comprises the following steps: in a muffle furnace, heating to a specific temperature under a corresponding protective atmosphere, cooling to roomtemperature, and crushing for use. The use method comprises the following steps: mixing the dephosphorizing agent in the preparation process, and reacting with the dephosphorizing agent to form a porous honeycomb structure on the surface of the dephosphorizing agent, thereby increasing the reaction interface area of the dephosphorizing agent and molten iron after adding the molten iron, improvingthe dephosphorizing interface conditions, promoting dephosphorization, enabling the dephosphorizing rate to be up to 93-97%, and greatly lowering the consumption of the dephosphorizing agent.