The invention relates to a solid-phase synthesis method for preparing a LiMn1-xFexPO4/C composite material. The preparation process comprises the following steps: amphiphilic organic matter (such as oleylamine, oleic acid, etc.) which is used as a surfactant is dissolved in a certain amount of a non-polar organic solvent (such as acetone, carbon tetrachloride, cyclohexane, etc.) to form a solution 1; the solution 1 is mixed with lithium salt, divalent manganese salt, ferrous salt, phosphate and a carbon source material at a certain stoichiometric ratio to obtain a mixture; and the mixture successively undergoes wet ball-milling, drying, pre-sintering and calcination so as to obtain the nanoscaleLiMn1-xFexPO4/C composite cathode material for a lithium ion battery. According to the preparation method, the amphiphilic surfactant and the non-polar organic solvent are introduced on the basis of a traditional soilid-phase synthesis method so as to prepare the LiMn1-xFexPO4/C composite material with the particle size of 5-60 nm. The invention provides a feasible scheme for large-scale preparation of the nanoscale lithium ferromanganese phosphate material.