The new unknown impurity appeared in the process of the synthesis of sildena citrate was purified, characterized and synthesized.The chlorine gas bubbled into chloroform solution of 5-(2-ethoxyphenyl) -1-methyl-3-propyl-1,6-dihydro-7H-pyrazolo[4,3-d]pyrimidin-7-one to synthesis 5-(5-chloro-2-ethoxyphenyl)-1-methyl-3-propyl-1,6-dihydro-7H-pyrazolo[4,3-d]pyrimidin-7-one, and the structure were confirmed by MS, NMR, IR.This impurity was not reported that literature researches and exptl. studies found that the impurity is one of the main factor to reduce the purity of sildenafil citrate in order to ensure the drug is safe and effective, strict control of the impurity content in the synthetic process should be taken seriously.This paper focuses on the characterization of the formation mechanism and structure of the impurity.