The invention discloses a method for preparing a Sm2O3 film by a solvent heat method. The method comprises the following steps of: adding analytically pure SmCl3.6H2O into isopropyl alcohol or a mixed solvent of the isopropyl alcohol and water, stirring, and thus obtaining solution A; adjusting the pH value of the solution A by using ammonia water to form coating film liquid; pouring the coating film liquid into a hydrothermal kettle, soaking a substrate into the coating film liquid, sealing the hydrothermal kettle, reacting at the hydrothermal temperature of between 120 and 200 DEG C for 12 to 48 hours, and naturally cooling to room temperature; and opening the hydrothermal kettle, taking the substrate out, washing by using absolute ethanol cleanly, drying in a vacuum drying oven, and thus obtaining the Sm2O3 optical film on the surface of the substrate. In the method, a reaction for preparing the Sm2O3 film is finished in a liquid phase at a time without crystallization thermal treatment at a later stage, so that the defects of curl, seasoning crack, grain coarsening of the Sm2O3 film which are possibly caused in the thermal treatment process, reaction of the film with the substrate or the atmosphere and the like are overcome; the method has simple process equipment, is low in reaction temperatures and makes high pressure easily generated, the obtained film has higher purity, and crystal grains grow controllably. The film which is uniform and compact, does not have visible defects and has higher bonding force and higher appearance quality can be prepared.