The invention discloses a low-temperature preparation method of rare earth n-borate. The preparation method comprises the following steps: (1) weighing required raw materials RE2O3 (wherein RE is one of rare earth elements Y, La, Gd or Sm) and H3BO3 according to a stoichiometric ratio; (2) weighing one of required fluxing agents NH4Cl, ZnCl2, MgCl1 or Li2CO3, wherein the usage amount of the fluxing agent is 5wt%-15wt% of mass of the raw materials; and the raw materials and the fluxing agent are uniformly mixed after weighing; (3) raising the temperature of a crucible containing the mixture to 600-700 DEG C in a gradient manner, sintering for 3h-5h, and then reducing the temperature to room temperature along with a furnace; (4) after grinding the sintered product, washing with water, stirring and carrying out suction filtration, and repeatedly washing for 3-5 times; and (5) placing the washed product in an electrothermal blowing dry box for drying, grinding and sieving to obtain the rare earth n-borate. According to the low temperature preparation method, RE2O3 and H3BO3 are used as the raw materials, one of NH4Cl, ZnCl2, MgCl2 or Li2CO3 is used as the fluxing agent, a low temperature solid phase method is adopted, the preparation method is simple and easy to operate, energy source is saved, the production cost is reduced, the treatment process is simple and convenient, the time is saved, and the labor is saved.