Abstract: Oral transmucosal drug delivery can be defined as the administration of drug through the oral mucosa to achieve systemic effects. It has the advantages of high bioavailability and rapid drug response. In this review, we introduce the physiology of oral mucosa, and analyze the factors affecting the pharmacokinetic properties of oral transmucosal drug delivery in detail, such as physiological barriers, different administration sites, physicochemical properties of drugs, dosage forms, and formulation strategies. In addition, we describe the methods to evaluate the pharmacokinetic properties of this delivery systems, including in vitro permeability studies, buccal absorption studies, in vivo pharmacokinetic studies and physiologically based pharmacokinetics (PBPK) modeling, which provide methods and reference for the development of oral transmucosal drug delivery systems.