NOVELTY - Synthesis of sildenafil analog includes adding chloroacetyl chloride into reaction flask, cooling, adding anhydrous aluminum chloride, stirring, and cooling with ice water; and adding first compound into another reaction flask, heating while stirring, adding into ice water mixture, precipitating solids, filtering, washing with water, filtering, and drying to obtain second compound; adding N,N-dimethylformamide, pyridine and second compound into another reaction flask, insulating, cooling, crystallizing, filtering, and collecting solids.    USE - Method for synthesis of sildenafil analog (claimed).    ADVANTAGE - The method is simple, environment-friendly and suitable for industrial production, achieves yield of more than 75% for each step, and it has high total yield.    DETAILED DESCRIPTION - Synthesis of sildenafil analog comprises:    (A) adding chloroacetyl chloride into reaction flask, cooling to below 20 degrees C, adding anhydrous aluminum chloride, stirring, and cooling with ice water; and adding first compound into another reaction flask, heating while stirring for 2-4 hours, adding into ice water mixture, precipitating solids, filtering, washing with water, filtering, and drying to obtain second compound;    (B) adding N,N-dimethylformamide, pyridine and second compound into another reaction flask, insulating for 0.5-1 hour, cooling to below 20 degrees C, crystallizing, filtering, and collecting solids; and adding potassium hydroxide into absolute ethanol, placing into another reaction flask with collected solids, heat-refluxing for 3-5 hours, cooling to room temperature, adjusting pH with hydrochloric acid, precipitating crystals, filtering, washing solid material with water, filtering, and drying to obtain third compound;    (C) putting thionyl chloride into reaction flask with reflux condenser, cooling with ice water to below 20 degrees C, adding third compound, and insulating for 1-3 hours; and vacuum-recovering thionyl chloride to dryness, adding tetrahydrofuran into reaction flask, cooling to below 20 degrees C, heat-refluxing for 2-4 hours, cooling to room temperature, filtering, and drying to obtain fourth compound; and    (D) adding pyridine into reaction flask with condenser, adding phosphorus pentasulfide and fourth compound, insulating for 1-3 hours, and vacuum-recovering pyridine at 70 degrees C; and steaming, slowly adding reaction flask with distilled water, crystallizing, filtering, and drying yellow solid material.    The first compound is 5-(2-ethoxy)phenyl-1-methyl-3-n-propyl-1,6-dihydro-7H-pyrazolo (4,3-d) pyrimidine-7-one. The second compound is 5-(2-ethoxy-5-(chloromethyl-1-ylcarbonyl)phenyl)-1-methyl-3-n-propyl-1,6-dihydro-7H-pyrazolo (4,3-d) pyrimidin-7-one. The third compound is 5-(2-ethoxy-phenyl)-1-methyl-5-carboxy-3-n-propyl-1,6-dihydro-7H-pyrazolo (4,3-d) pyrimidin-7-one. The fourth compound is 5-(2-ethoxy-5-(4-methyl-piperazin-1-yl-carbonyl)phenyl)-1-methyl-3-n-propyl-1,6-dihydro-7H-pyrazolo (4,3-d) pyrimidin-7-one. The sildenafil analog is 5-(2-ethoxy-5-(4-methyl-piperazin-1-yl-thiocarbonyl)phenyl)-1-methyl-3-n-propyl-1,6-dihydro-7H-pyrazolo (4,3d) pyrimidine-7-thione.