Filtrate reducer for viscoelastic fracturing fluid and preparation method of filtrate reducer

The invention provides a filtrate reducer for viscoelastic fracturing fluid and a preparation method of the filtrate reducer. The filtrate reducer comprises the following components in parts by weight: 5-10 parts of sodium alginate, 4-8 parts of a soft monomer, 4-8 parts of a hard monomer, 1-3 parts of humic acid, 0.5-0.7 part of an initiator, 0.1-0.2 part of a cross-linking agent, 3-5 parts of nano calcium carbonate and 100 parts of water. According to the invention, a sulfonic acid monomer and an acrylic acid monomer are compounded as a soft monomer, an anionic property is provided, a solvation layer is formed in a polymer chain link, and certain water solubility and pollution resistance are achieved; and the molecular motion resistance is improved by using the hard monomer, so that the effects of enhancing the thermal stability and resisting high temperature are achieved. The calcium resistance is improved by using sodium alginate; the natural mineral source sodium humate is introduced, so that the hydroph