The invention relates to the field of preparation of soil conditioners, in particular relates to a soil conditioner for preventing and treating clubroot of oilseed rape as well as a preparation methodand application of the soil conditioner. The soil conditioner for preventing and treating clubroot of the oilseed rape, which is provided by the invention, comprises the following raw materials in parts by weight: 70 to 80 parts of water-soluble calcium, 0.1 to 1 part of water-soluble magnesium, 2 to 3 parts of organic nitrogen, 8 to 15 parts of humus, 6 to 15 parts of an adhesive and 20 to 25 parts of a sustained-release agent. The soil conditioner for preventing and treating clubroot of the oilseed rape does not contain chemical pesticide, is non-toxic and harmless for crops, avoids environmental pollution, is favorable for environmental protection and food safety, can be used without being influenced by the geological condition and the soil environment, has a prominent and stable prevention and treatment effect on the clubroot, is not liable to cause soil hardening and environmental pollution, reduces the pathogen quantity of the soil fundamentally, prevents diseases from the source, prevents and treats the clubroot, and has the effects of improving the soil fertility and promoting the growth of crops; and the water-soluble calcium in the conditioner is releases slowly, so non-uniform distribution of Ca<2+> in the soil is avoided.