The invention discloses a synthesis method of Anagliptin key intermediate. The synthesis method comprises the following steps: jointing cyanoacetaldehyde with N,N-dimethylformamide dimethyl acetal to obtain (2E)-3-(dimethylamino)-2-formylacrylonitrile, further performing ring closing with 3-amino-5-methylpyrazole to obtain 2-methyl-pyrazolo[1,5-a]pyrimidine-6-carbonitrile, and then hydrolyzing to obtain the Anagliptin key intermediate. The invention relates to a brand-new method for synthesizing 2-methyl-pyrazolo[1,5-a]pyrimidine-6-carboxylic acid, and the method has the advantages of use of raw materials, less side products, high product purity, and low whole cost.