The invention discloses a method for synthesizing lithium ferrous phosphate in a solution, which relates to an anode material of a lithium ion battery, and the method belongs to the technical field of chemical material preparation. The method comprises the following steps: reacting phosphoric acid, iron trioxide with reduced iron powder at a certain temperature, adding a lithium source and a carbon source, continuing the reaction to obtain a mixing solution, and filtering, drying and calcining the mixing solution to obtain the lithium ferrous phosphate. Compared with a synthetic method of using the reduced iron powder as an iron source, the method adopts the iron trioxide as most of iron sources so as to reduce the cost of raw materials greatly; the obtained product is uniform in component and good in batch stability, and the synthesized lithium ferrous phosphate material serving as the anode material of the lithium ion battery has the excellent electrochemistry performance; 3C multiplying power discharging specific capacity is more than 130mAh/g at room temperature; the synthesized lithium ferrous phosphate material is suitable for being used as power lithium ion batteries; and the method has short sintering period, simple production process, less energy consumption and no pollution, such as tail gas and waste liquor, and is suitable for mass industrial production.