A dual-arm benzoic-acid organic rare-earth efficient luminescent material and a preparation method thereof are disclosed, the structural formula is Ln2 (BCMT) 2 (H2O) 4-2n (L2) n. 1,4-ethylene glycol terephthalate is added into a mixed solvent of ethanol and N, N-dimethylformamide or a mixed solvent of ethanol and dimethylsulfoxide for stirring into a homogeneous solution 1; LnCl3 or Ln (NO3) 3 is added into ethanol to obtain a LnCl3 or Ln (NO3) 3 ethanol solution, the LnCl3 or Ln (NO3) 3 ethanol solution is added into the solution 1 to obtain a reaction solution 2; L2 is added into ethanol to obtain a L2 ethanol solution, the L2 ethanol solution is stirred until the solution is transparent, the transparent L2 ethanol solution is added into the reaction solution 2 for reaction, and then the dual-arm benzoic-acid organic rare-earth efficient luminescent material is obtained by cooling, suction filtration and washing. In the long arm benzoic-acid organic rare-earth efficient luminescent material, the molecular structure comprises a flexible bond C-O bond, so that ligand-terminal-COOH has better coordination space with rare earth ions, meanwhile by ester group mutual coordinating role, a prepared organic rare earth complex has a more excellent large flat molecular structure, high fluorescence intensity and long fluorescence lifetime.