The present invention provides a drilling fluid filtrate reducer, which contains, by weight, 30-70 parts of an oil phase, 10-30 parts of a nonionic surfactant, and 20-50 parts of a nanometer silica and nanometer calcium carbonate water dispersion. The invention further provides a preparation method of the drilling fluid filtrate reducer, wherein the preparation method comprises: mixing a nonionic surfactant and an oil phase according to a certain ratio, carrying out high speed stirring for 10-30 min at a temperature of 20-40 DEG C with a magnetic stirrer, gradually adding a nanometer silica and nanometer calcium carbonate water dispersion under low speed stirring, and carrying out high speed stirring for 10-30 min to obtain the drilling fluid filtrate reducer. According to the present invention, the pore and the micro-crack are blocked by using the nanoparticles, the formed filter cake is relatively compact, the quality of the filter cake can be improved, the filtrate reducing of the drilling fluid can be reduced, the stability of the borehole wall can be improved, and the good oil and gas layer protection effect is provided.