The invention relates to a soil improving method for rectangular-pipe jacking construction in a gravel soil layer. The soil improving method specially includes that a PAM (polyacrylamide) solution and a bentonite mud solution are added to a working surface and the soil body of a soil pressure bin, content of the PAM in the PAM solution ranges from 1g/L to 4g/L, and the mass concentration of the bentonite mud solution ranges from 7% to 11%. The soil body cut with the soil improving method is improved to have the advantages of good fluidity, low frictional angle, middle compressibility, lower adhesivity, low permeability and the like, the full-tank jacking and jacking construction speed are ensured, problems about uneasiness in soil pressure balance construction, severe abrasion of a nose cutter, easiness in 'caking', proneness to 'water gushing' when the nose cutter passes through the water-baring layer and the like can be solved, requirements of the rectangular-pipe jacking construction with the soil pressure balance can be met, and technical support is provided for establishment of dynamic soil pressure balance of the soil-pressure-balance rectangular-pipe jacking working surface of a multicutter in the gravel soil layer.