The invention belongs to the technical field of agriculture soil conditioning agent production and in particular relates to a soil conditioning agent for rapid and long-term repairing of farmland cadmium pollution and a preparation method of the soil conditioning agent. The soil conditioning agent comprises the following raw materials in parts by weight: 60-80 parts of a silicon calcium potassiummagnesium fertilizer, 5-10 parts of anhydrous calcium sulfoaluminate, 5-15 parts of alums and 10-15 parts of sodium carbonate. The silicon calcium potassium magnesium fertilizer comprises plant absorbable nutrition components, and the mass percentages of the nutrition components are as follows: greater than or equal to 30% of effective silicon, greater than or equal to 28% of effective calcium, greater than or equal to 4% of effective magnesium and greater than or equal to 5% of effective potassium. By adopting the soil conditioning agent, a heavy metal cadmium in polluted farmland soil can berapidly passivated, the migration and the toxicity of the heavy metal cadmium can be reduced, and ions and groups for forming solid precipitate substances with the heavy metal cadmium can be slowly released for a long time, so that the problem of repairing ineffectiveness caused by environment change can be avoided, and problems such as soil caking and salinization caused by long-term massive application of a heavy metal passivator can be also avoided.