The invention provides a basic farmland soil modifying agent, belonging to the technical field of fertilizer production. The modifying agent is prepared by adopting grass carbon, dried pine straw, vermiculite, natural zeolite, humus soil, well-rotted farmyard manure, aluminum sulfuric acid, zinc sulfuric acid and manganese sulfuric acid as raw materials. The preparation method is to crush the grass carbon, the dried pine straw, the vermiculite and the natural zeolite and mix with other raw materials to be uniformly stirred. The basic farmland soil modifying agent can loosen soil of basic farmland, improve the physical character and chemical character of the basic farmland soil, promote crop growth, improve utilization rate of fertilizer, and increase yield of crops.