The invention relates to a method for preparing the cathode material used in a ferrous phosphate lithium battery by adopting low-valent oxyacid of phosphorus; the technical proposal is as follows: lithium salt, ferrous salt, phosphate, and hypophosphorous acid or hypophosphite are blended according to the mol ratio that Li: Fe: PO4<3->: H3PO2 or AH2PO2 or E(H2PO2)2 is equal to x: y (1-z): k. Carbon-containing compound or carbon powder and wet milling liquid are added, ball milling is carried out for 3-12 hours, and the mixture is dried in atmospheric pressure or vacuum at 48-100 DEG C. The dried powder is prepared into the lithium iron phosphate containing controllable Fe2P by two-stage sintering process or temperature programmed two-stage sintering process. When the molecular formula of hypophosphite, one ingredient of the reaction composite, is AH2PO2 or E(H2PO2)2, A is Li<+>, Na <+>, K<+>, Ag<+> or NH<+>, E is Ca<2+>, Sr<2+>, Ba <2+>, Ga <2+>, Ge<2+>, Sn<2+>, Sc<2+>, Mn<2+>, Fe<2+>, Co<2+>, Ni<2+>, Cu<2+>, Zn<2+> or Mo<2+>. The method of the invention has low cost of raw materials, broad source of raw materials, simple preparation and short time consumed; the prepared materials have even composition, excellent discharge performance and good discharge cycling performance in heavy current.