The soluble biphenyl polyimide was synthesized by a two-step method using 3,3’,4,4’-biphenyl tetracarboxylic dianhydride（BPDA）as dianhydride monomer and side-chain 4,4""-diamino-3,3""-dimethyldiphenylmethane（DMMDA）as diamine monomer. The effects of monomer ratio, solid content, reaction temperature, and reaction time on the molecular weight of polyamide acid (PAA), and the effect of chemical imidization time on the degree of polyimide(PI) imidization were discussed. The optimum synthesis conditions of polyimide were determined, for the synthesis of polyamide acid, the optimal monomer ratio is determined to be 1:1, the solid content is 12%, the reaction temperature is 20 ℃, and the reaction time is 8 h. The characteristic viscosity of polyimide (PAA) under the optimal conditions was 1.88 dL/g. For the synthesis of polyimide, the optimal chemical imidization time is 24 h, and the degree of imidization is 98% under these conditions. Afterwards, a series of structures and properties of polyimide synthesized under the optimum conditions were characterized. The experimental results show that the synthesized polyimide has a higher molecular weight (the number-average molecular weight M_n and weight-average molecular weight M_w are 1.4×105 and 2.1×105, respectively) and narrower distribution. While maintaining the great resistance to heat and solvent, the processing performance of the polyimide is improved, such as film formation and solubility, so it is ideal material to prepare nanofiltration membranes.