To find selective inhibitor of phosphodiesterase type 5 (PDE5), the essential structure elements of clinically used drugs sildenafil, vardenafil, and tadalafil were combined and a tetracyclic parent was constructed to which in 2-positions substituted acetic acid methylesters were introduced to form 17 novel vasodilators, methyl (11aS)-1,2,3,5,11,11a-hexahydro-3,3-dimethyl-1-oxo-6H-imidazo[3',4':1,2]- pyridin[3,4-b]indol-2-substituted acetates. By molecular field analysis (MFA), an equation of three-dimensional quantitative structure-activity relationship (3D QSAR) was established, which not only revealed the dependence of the in vitro vasorelaxation activities on the structures but also pointed out the way to design new lead compounds properly. Docking these novel vasodilators into the hydrophobic pocket of phosphodiesterase type 5 (PDE5) revealed that their adaptabilities to this pocket did significantly affect on their vasorelaxation activity. Actually, the docking adaptabilities of these novel vasodilators to PDE5 were consistent with the conformational requirements of them to MFA and with the crystal conformation of two representatives.