Environment-friendly filtrate reducer for water-based drilling fluid and preparation method of filtrate reducer

The invention provides a filtrate reducer for environment-friendly water-based drilling fluid and a preparation method of the filtrate reducer. The modified diatomite is prepared from the following raw materials: diatomite, starch, an alkenyl monomer, a coupling agent, a cross-linking agent and an initiator. The content of the diatomite is 0.2 to 3.0 parts by mass, the content of the starch is 2 to 10 parts by mass, the content of the alkenyl monomer is 0.2 to 3.0 parts by mass, the content of the coupling agent is 6.5 parts by mass, the content of the cross-linking agent is 0.05 to 1.0 part by mass, and the content of the initiator is 0.02 to 0.25 part by mass. According to the filtrate reducer disclosed by the invention, the apparent viscosity and the plastic viscosity of a system are not obviously changed while the filter loss of the environment-friendly water-based drilling fluid is reduced, namely, the influence on the rheological property of the drilling fluid is relatively small. The diatomite, the sta