The invention relates to an improved synthesis process for sildenafil which is a raw medical material for treating male erectile dysfunction (ED). The improved synthesis process is characterized by using different synthetic routes, a compound (I) is subjected to N-methyl piperazine reaction in water, crystallization is directly realized through adjustment of the pH value to obtain a compound (IV), the reaction is rapid, and no by-product is generated; the compound (IV) is subjected to reaction with a water-mixed solvent under the alkaline condition, and after the reaction is finished, crystallization is directly realized through adjustment of the pH value by addition of diluted hydrochloric acid, so as to obtain the pure sildenafil. The synthesis process for the prepared sildenafil is simple and convenient in operation, mild in reaction condition, high in yield and purity, and suitable for industrial production.