Method for weighting drilling fluid by using low-density weighting agent

The invention discloses a method for weighting drilling fluid by using low-density weighting agents, which is characterized in that the low-density weighting agents matched with different size fractions are selected and used according to the particle size distribution of the drilling fluid: (1) for the drilling fluid with the particle size distribution Sw lower than 1500m < 2 >/kg, the proportions of the weighting agents with various fineness are adjusted according to the mass ratio of 325-200 meshes to 1000-600 meshes to 1500 meshes of 5: 3: 2; and (2) for the slurry with the particle size distribution Sw equal to or higher than 1500m < 2 >/kg, adjusting the proportions of weighting agents with various fineness according to the mass ratio of 325-200 meshes to 1000-600 meshes to 1500 meshes of 7: 2.5: 0.5. The problems are solved through methods/treating agents such as composite inhibition, flow pattern adjustment, size fraction matching and mud cake quality adjustment, and meanwhile the stability of a drilli