The invention relates to an application of tetrahydropyrimidine and derivatives thereof in preparing oral absorption enhancers. Found by the inventors under study, 1,4,5,6-tetrahydro-2-methyl-4-pyrimidinyl carboxy (tetrahydropyrimidine) and derivatives thereof have very strong oral absorption enhancing effect and higher safety, so that the tetrahydropyrimidine and derivatives thereof can be used for preparing oral absorption enhancers. The invention also provides an oral absorption enhancer containing the tetrahydropyrimidine or the derivatives thereof as active components. Further, the invention provides a composition for oral administration containing the oral absorption enhancers. Proven by experiments, the oral absorption enhancer provided by the invention has obvious oral absorption enhancing effect.