The invention relates to oil based drilling fluid used in oilfield drilling, in particular to a polymer filtrate reducer and a preparation method. The preparation method includes steps of uniformly mixing 100 parts of lignite, 40-50 parts of organic amine, 10-20 parts of polybutadiene rubber and 1-2 parts of EDTA(ethylenediaminetetraacetic acid), reacting for 4-5 hours at the temperature of 220-260 DEG C, drying and crushing to obtain dark-brown solid powder. The preparation method is simple; and the filtrate reducer prepared by the preparation method is good in performance, high-temperature resistant and salinity-resistant, good in reduction performance, capable of keeping rheological property of the drilling fluid and capable of protecting a hydrocarbon reservoir against collapsing and lowering filtrate reduction quantity of the drilling fluid.