The invention discloses an application method of a chemical fertilizer in the fertilization reduction synergy of oilseed rape. The application method comprises the following steps: A, the type and the application amount of fertilizers are determined: the oilseed rape fertilizing amount is the total fertilizing amount of a nitrogen fertilizer (by pure nitrogen), a phosphorus fertilizer (by phosphoric pentoxide) and a potassium fertilizer (by potassium oxide), the use amount of the chemical fertilizer urea and the use amount of a nitrogen fertilizer synergist are calculated according to the total nitrogen amount of the chemical fertilizer, the use amount of a phosphorus activator is calculated according to the oilseed rape field plantation area, and the total phosphorus and potassium amount of the organic fertilizer is calculated; B, the organic fertilizer, the phosphorous fertilizer, the potassium fertilizer, the nitrogen fertilizer synergist and the phosphorus activator are uniformly once applied as a base fertilizer; C, the nitrogen fertilizer is applied in three stages, the phosphorus fertilizer and the potassium are once applied as a base fertilizer, a use amount ratio of the nitrogen fertilizer base fertilizer to a seedling fertilizer to a stem elongation fertilizer is (4-7):(1-3):(1-3), the base fertilizer is applied in furrows, the seedling fertilizer is applied in 10-40 days after oilseed rape seedlings are transplanted, and the stem elongation fertilizer is applied in 50-100 days after the oilseed rape seedlings are transplanted; and D, fertilization is not carried out in the post-emergence stage, the flowering stage or the maturation stage. The method has the advantages of easiness in implementation, simplicity in operation, reduction of the application of the chemical fertilizer, improvement of the quality of soil through applying the organic fertilizer, improvement of the nutrient utilization efficiency and increase of the yield of the oilseed rape.