A simple, rapid and accurate method for quantitative analysis of a highly selective phosphodiesterase-5 inhibitor (PDE5), TPN171 by high performance liquid chromatography and tandem mass spectrometry in human plasma was proposed and validated successfully using D3-TPN171 as internal standards (ISTD). An aliquot of 100 μL of plasma was mixed with internal standard and was precipitated with acetonitrile. Gradient elution was performed on a ACQUITY HSS T3 column (50 × 2.1 mm, 1.8 μm) coupled with a ACQUITY column in-line filter at 40℃, by 5 mM ammonium acetate in water containing 0.1 % formic acid and 0.1 % formic acid in acetonitrile as the mobile phase. The total analytical run time was 3.5 min. The analyte was monitored using multiple reaction monitoring (MRM) scan in positive polarity mode. The ion transition was m/z 442.2→113.2 and 445.2→116.2 for TPN171 and D3-TPN171 respectively. The method was validated for specificity, sensitivity, precision, accuracy, and other analytical parameters. The results found were satisfactory over the linear calibration range of 1-500 ng/mL. Within-day precisions ranged from 1.8 to 7.3 %, and between-day precisions from 2.3 to 4.9 %, accuracies were 95.5-99.8 %.The validated method was successfully applied to determine the plasma concentration after oral administration of 10 mg TPN171 in six healthy volunteers.