The invention relates to a preparation method of lithium iron phosphate for the carbon cladding of a lithium ion battery. The prior lithium iron phosphate preparation technical art is complex, and has high cost. The invention has the synthetic process that: ferric oxide, phosphoric acid, simple organics and doped element compound are mixed and dried, the mol ratio of phosphate radical ion, ferric ion and doped element ion is 1:y:z, wherein, y is larger than or equal to 0.95 and smaller than or equal to 1, and y plus z is equal to 1; the mixture is added with lithium source compound, added with water to be mixed and dried, the mol ratio of lithium ion and phosphate radical ion is x:1, and x is larger than or equal to 0.95 and smaller than or equal to 1.05; the mixture which reacts for 2 to 20 hours under 500 to 800 DEG C is cooled in a furnace. The invention finally produces the precursor uniformly mixed with superfine crystal grain, and during the subsequent high temperature solid phase reaction, the end product lithium iron phosphate can be produced through shorter distance diffuseness of atoms. The end product has high purity, the crystallisation is good, the capacity is high, and the cycle stability is good.