The soluble biphenyl polyimide was synthesized by a two-step method using 3,3’,4,4’-Biphenyl tetracarboxylic dianhydride（ BPDA）as dianhydride monomer and a 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, and the effect of chemical imidization time on the degree of PI imidization were discussed. The optimum synthesis conditions of polyimide were determined : for the synthesis of polyamide acids, the optimal monomer ratio was determined to be 1:1, the solid content was 12%, the reaction temperature was 20 ℃, and the reaction time was 8 h. The characteristic viscosity of polyimide (PAA) under the optimal conditions was 1.88dL g-1. For the synthesis of polyimide, the optimal chemical imidization time was 24h, and the degree of imidization was 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 140,000 and 210,000, respectively) and narrower distribution. While maintaining great resistance to heat and solvent, the processing performance of the polyimide is improved, such as film formation and solubility, so it is ideal for preparing nanofiltration membranes as material.