The invention relates to a farmland soil heavy metal degradation agent, which is composed of the following components by weight: 4-5% of liquid peptidase; 20%-30% of zeolite powder or attapulgite mineral powder; 5%-10% of a seaweed extracting solution; 40%-50% of waste Chinese herbal medicine residues (80%-90% of dry organic matters); 3-4% of plant amino acid; 0.5%-1% of wine yeast and a microbial agent; 1.5%-2% of biochar or activated carbon; the liquid peptidase is a compound formed by connecting amino acids through peptide bonds and is composed of active enzyme bacteria. The soil conditioner has the advantages of controlling excessive heavy metal cadmium absorption of crops, improving soil, enhancing organic matters and repairing lack of trace elements in the soil, and has a good effect on improving soil performance.