The invention provides an agent for farmland heavy metal pollution treatment and a preparation method thereof, and relates to the technical field of soil remediation and treatment. The medicament mainly comprises the following raw materials: lime, kaolin, sepiolite and vinasse. According to the invention, the biochar prepared from vinasse is used as a main porous substance, so that the biochar has a relatively good adsorption effect on heavy metals, and lime, kaolin and sepiolite can be compounded, so that the competitive effect of various heavy metals is reduced, and the passivation effect of heavy metals in farmland soil is improved. Meanwhile, the lime and the kaolin have viscosity, so that the molecular interaction between the lime and the kaolin and the vinasse biochar is relatively strong. In addition, according to the preparation method of the agent, acid modification and sintering are mainly adopted, after minerals are modified through acid liquor, the passivation effect of the agent is enhanced, then after mixing, sintering is conducted under the high-temperature condition, the combination effect of the raw materials is good, and the passivation effect of the agent can be further improved.