The invention discloses a hydro-thermal synthesis method for a positive material for lithium ion batteries by using siderite. The hydro-thermal synthesis method comprises the following steps: firstlydissolving siderite into dilute phosphoric acid to obtain an iron solution, and then carrying out a hydrothermal reaction on the iron solution and lithium hydrate to obtain the positive material for lithium ion batteries such as lithium iron phosphate. According to the hydro-thermal synthesis method, phosphoric acid is directly added into siderite, iron of siderite is directly dissolved into phosphoric acid by controlling the concentration of phosphoric acid and the temperature and the time of the reactor, then hydrogen peroxide is added to obtain lithium iron phosphate through the hydrothermal reaction, the lithium iron phosphate after the reaction exists in a sediment form, and the positive material for lithium ion batteries such as lithium iron phosphate can be obtained through direct filtering, so that the working procedure of leaching iron in siderite by using hydrochloric acid or sulfuric acid in advance is omitted, the cost is reduced, other impurities unbeneficial to battery performances are not introduced, and the electrical conductivity of the batteries is improved.