The invention discloses a preparation method of modified lithium ferric manganese phosphate. The preparation method comprises steps as follows: an iron fluo-silicate solution and a potassium permanganate solution are subjected to a reaction, an iron permanganate solution is obtained and subjected to concentration and crystallization, crystals are obtained and subjected to ball-milling with cell-grade lithium phosphate and glucose by adding ethanol, an obtained mixture is sintered, a material obtained after sintering is washed, dried, broken, screened and subjected to iron removing, and a product is obtained. According to the preparation method of modified lithium ferric manganese phosphate, the process is simple, strict control for the ratio of manganese to iron can be realized, batch stability is controllable, meanwhile, uniform doping of an added modifier can be realized, and tap density and energy density of the product are high.