The invention discloses a dry powder extinguishing agent and a preparation method. The dry powder extinguishing agent is prepared by the following main raw materials in parts by weight: 25-35 of alkali slag, 30-42 of sodium bicarbonate, 17-26 of sodium oxalate, 5-12 of marl, 4-9 of magnesium carbonate powder, 2-5 of silicone oil, 6-14 of gypsum, 2.5-5 of chlorinated rubber, 2-6 of zinc borate, 1-4 of polybutylene, and 1.5-4 of tin oxide. The dry powder extinguishing agent is excellent in extinguishing performance of three-dimensional combustion fire, high in extinguishing efficiency, high in moisture resistance, excellent in recrudescence resistance and environment-friendly.