The invention belongs to the technical field of pharmaceuticals, and particularly relates to a phosphodiesterase inhibitor shown in the formula I, salt, solvent compounds, polymorphic substances and isomers acceptable in the pharmaceuticals. The invention further relates to compound drug preparations, drug compounds and application thereof. The compound related to the invention can be applied to relevant disease treatment of abnormal expression mediation of phosphodiesterase 9 (PDE 9).The description is shown in the formula I.