The invention discloses a preparation method of Idelalisib and an intermediate thereof. The method comprises the following steps: using 2-halogeno-6-fluorine-N-phenyl benzamide as a raw material, performing a reaction on the 2-halogeno-6-fluorine-N-phenyl benzamide and N-(9'-tetrahydropyrane-6'-purinyl)-L-2-aminobutyric acid, performing a reaction on a reaction product and ammonium carbonate or liquid ammonia to replace 2-halogeno with ammonia by cuprous oxide, copper and potassium carbonate or cesium carbonate, performing a ring-closure reaction under the existence of hexamethyl disilazane and iodine, and performing a reaction on a reaction product and hydrochloric acid solution to remove pyran finally, so as to generate an Idelalisib finished product. According to the process, the novelmethod is provided for synthesizing the Idelalisib; in the process, the easily available 2-halogeno-6-fluorobenzoic acid is used as a starting material, and zinc acetate pollution and the like can beprevented from being generated through reduction of nitro and the like with zinc powder in a synthesis process.