The invention relates to a preparation method of a liquid crystalline polyimide solution. The preparation method comprises the following steps of: mixing P2O5 and polyphosphoric acid under the protection of nitrogen gas at 100-150 DEG C, stirring to obtain a polyphosphoric acid solution of P2O5, and cooling the polyphosphoric acid solution to 100-120 DEG C; and (2) adding a diamine monomer containing heterocyclic rings and a dianhydride monomer into the polyphosphoric acid solution to obtain a reactant with the solid content of 5-20wt%, raising the temperature to 120-220 DEG C under the protection of nitrogen gas and reacting for 4-6h to obtain the liquid crystalline polyimide solution. The liquid crystalline polyimide solution is prepared on the basis of realizing the solubility of polyimide, which lays the technological basis for further preparing high-performance polyimide fiber or film; and the preparation method is simple in process, low in cost, capable of solving the problem that a phenol solvent has great influence to the environment and favorable in application prospect.