The invention relates to a method for preparing biological diesel oil with a low solidifying point by biological fermentation coupling. The method comprises the following steps of: adding active sludge containing a lipoclasis strain into raw material grease, performing shaking culture, and further standing for layering, wherein the upper layer is of fatty acid; adding methanol and an acidic catalyst for performing esterification reaction; then performing standing separation on water-containing methanol steam generated during the process and mixed products of the esterification reaction, performing distillation and refining on the obtained biological diesel oil in the upper layer, and removing methanol and residues to get the biological diesel oil. According to the method disclosed by the invention, lipase generated by metabolism of the lipoclasis strain is utilized for decomposing grease into fatty acid taking C10-C16 as main components, the biological diesel oil with shorter carbon chain in comparison with the traditional biological diesel oil and lower solidifying point can be prepared after esterification reaction, the product has the advantages of small viscosity and good low-temperature mobility, a preparation process is mild, the reaction for shortening the carbon chain is performed at the very low temperature, and the energy consumption is low.