The invention relates to a pretreatment method of a boiling black liquor in diffusion dialysis recovery pulp manufacturing, and belongs to the technical field of pulp manufacturing. The pretreatment method comprises: adding a flocculating agent into the boiling black liquor with the concentration of sodium hydroxide being 8-30 g/L for flocculation to make the concentration of the flocculating agent to be 5-10 mg/L, then carrying out filtration treatment with a micropore filtering membrane having a pore size of 0.1-1 [mu]m and further carrying out filtration treatment with an ultrafiltration membrane having a pore size of 20-1000 angstrom for reducing the concentration of large grained impurities and recycling lignin, and obtaining a pretreatment liquid for a cation-exchange membrane to carry out diffusion dialysis treatment. According to the pretreatment method, the specific pretreatment method is adopted for treatment of the boiling black liquor, then the treated boiling black liquor gets into diffusion dialysis process, so that the recycling rate of alkali and hemicellulose in the boiling black liquor can be improved, diffusion dialysis membrane system damages caused by an alkali liquor can be reduced, blockages of a diffusion dialysis membrane can be prevented, and service life of the diffusion dialysis membrane can be improved, and the pretreatment method can ensure that diffusion dialysis and subsequent processes are carried out smoothly.