A method for synthesizing crystalline form A of prasugrel, wherein the crystalline form A has the following X-ray powder diffraction pattern measured by copper cathode diffractometer, expressed as the interplanar spacing d, Bragg angle 2θ, intensity and relative intensity, wherein the relative intensity is expressed as a percentage relative to the strongest ray: 2θ angle (°) interplanar spacing d(Å) intensity relative intensity (%) 7.639 11.56 1569 19 9.717 9.09 192 2.3 11.118 7.95 2838 34.4 13.340 6.63 8253 100 13.679 6.47 808 9.8 14.360 6.16 4184 50.7 14.639 6.05 5807 70.4 14.980 5.91 2125 25.7 17.737 5.00 720 8.7 18.480 4.80 2087 25.3 18.761 4.73 8089 98 19.221 4.61 6421 77.8 20.640 4.30 707 8.6 21.301 4.17 7910 95.8 22.298 3.98 1681 20.4 22.640 3.92 1743 21.1 23.300 3.81 7209 87.4 24.258 3.67 6855 83.1 24.682 3.60 649 7.9 26.221 3.40 1237 15 26.862 3.32 1947 23.6 28.320 3.15 501 6.1 29.422 3.03 548 6.6 30.179 2.96 323 3.9 31.280 2.86 2091 25.3 31.922 2.80 632 7.7 32.780 2.73 924 11.2 33.421 2.68 362 4.4 34.960 2.56 531 6.4 35.557 2.52 218 2.6 36.960 2.43 486 5.9 38.060 2.36 583 7.1 38.339 2.35 358 4.3 39.000 2.31 953 11.5 characterized in that the method comprises the following steps: o-fluorobenzyl cyclopropyl ketone as a raw material is reacted with 1,3-dibromo-5,5-dimethylhydantoin or 1,3-dichloro-5,5- dimethylhydantoin as a halogenating agent in acetic acid as a solvent to give α-cyclopropylcarbonyl-2-fluorobenzyl halide (compound 2); α-cyclopropylcarbonyl-2-fluorobenzyl halide is reacted with 2-oxo-4,5,6,7-tetrahydrothieno[3,2-c]pyridine toluenesulfonate (compound 3) in the concerted catalysis of a phase transfer catalyst and an inorganic salt using an inorganic base as an acid binding agent and DMF as a solvent under nitrogen atmosphere to get 5-(α-cyclopropylcarbonyl-2-fluorobenzyl)-2-oxo-2,4,5,6,7-tetrahydrothieno[3,2-c]pyridi ne (compound 4); 5-(α-cyclopropylcarbonyl-2-fluorobenzyl)-2-oxo-2,4,5,6,7-tetrahydrothieno[3,2-c]pyridine is acylated in the presence of an acylating reagent using triethylamine as an acid binding agent and DMF as a solvent under nitrogen atmosphere at room temperature, and finally extracted, dried and filtrated to give the mother liquor of prasugrel, and the mother liquor is concentrated to get prasugrel as a gum; an alcoholic solvent is added to the prasugrel as a gum obtained in the above step, and crystal is precipitated after stirring at room temperature, then the crystal is filtrated to give prasugrel as solid; the reaction scheme is as follows: o-fluorobenzyl cyclopropyl ketone as a raw material is reacted with 1,3-dibromo-5,5-dimethylhydantoin or 1,3-dichloro-5,5- dimethylhydantoin as a halogenating agent in acetic acid as a solvent to give α-cyclopropylcarbonyl-2-fluorobenzyl halide (compound 2); α-cyclopropylcarbonyl-2-fluorobenzyl halide is reacted with 2-oxo-4,5,6,7-tetrahydrothieno[3,2-c]pyridine toluenesulfonate (compound 3) in the concerted catalysis of a phase transfer catalyst and an inorganic salt using an inorganic base as an acid binding agent and DMF as a solvent under nitrogen atmosphere to get 5-(α-cyclopropylcarbonyl-2-fluorobenzyl)-2-oxo-2,4,5,6,7-tetrahydrothieno[3,2-c]pyridi ne (compound 4); 5-(α-cyclopropylcarbonyl-2-fluorobenzyl)-2-oxo-2,4,5,6,7-tetrahydrothieno[3,2-c]pyridine is acylated in the presence of an acylating reagent using triethylamine as an acid binding agent and DMF as a solvent under nitrogen atmosphere at room temperature, and finally extracted, dried and filtrated to give the mother liquor of prasugrel, and the mother liquor is concentrated to get prasugrel as a gum; an alcoholic solvent is added to the prasugrel as a gum obtained in the above step, and crystal is precipitated after stirring at room temperature, then the crystal is filtrated to give prasugrel as solid;