Abstract: The present disclosure relates to an iron-based composite phosphate cathode material and a preparation method thereof, and a cathode plate and a sodium ion battery. The method for preparing an iron-based composite phosphate cathode material includes: uniformly mixing a sodium source, a phosphorus source, a carbon source, and water, and then mixing same with iron phosphate to obtain a first mixed system; and grinding the first mixed system to obtain a second mixed system, and then drying and sintering same. In the first mixed system, the total mass of Na element, Fe element, (PO4)3?, and the carbon source is 30%-40% of the mass of the water; and the viscosity of the second mixed system is greater than or equal to 300 Pa·S. According to the method, a solid reactant can be solubilized during grinding, the iron-based composite phosphate cathode material is uniformly sized nano spherical particles.