Abstract: With the declustering potential increasing, the collision-induced dissociation of the quasi-molecular ions of sildenafil took place before the ions enter Q₁, and the primary fragment ions were generated. With the fragment ions as precursor ions respectively, information of the ESI-MS³ ions was acquired by mass-spectrum analysis in MS² mode with a tandem mass spectrometer， and the fragmentation behavior of sildenafil was discussed. The 4 main fragment ions generated in the fragmentation were confirmed. Based on the determination of these 4 ions, a method of liquid chromatographycoupled with electrospray ionization-multi-reaction monitoring was established for the verification of sildenafil. The method has a highly sensitivity， and the results are clear and easy to make a judgement. Having more than 4 identification points required in an official document, the method fully meets the requirement for the verification of sildenafil added in Chinese traditional patented medical preparation and dietary supplements.