The invention relates to a method for microbial remediation of underground water petroleum hydrocarbon contamination by regulating soil buffer capability, which comprises detecting the soil particle size of contaminated site soil, dividing the contaminated site soil into coarse-grained soil and fine-grained soil; dividing the contaminated site soil into high buffer capacity soil and low buffer capacity soil; and adjusting the composition and ratio of a biostimulant solution added to the contaminated site soil based on the classification of the contaminated site soil. The detecting step includes classifying soil with a particle size between 0.075 mm and 60 mm and a mass greater than or equal to 50% of the total mass as coarse-grained soil; and classifying soil with a particle size not greater than 0.075 mm and a mass greater than or equal to 50% as fine-grained soil.