Abstract: Provided in the present invention is a pharmaceutical composition of a small molecular GLP-1R receptor agonist suitable for oral administration. More specifically, the present invention relates to a pharmaceutical composition containing (S)-2-(3S,8S)-3-(4-(3,4-dichlorobenzyloxy)phenyl-7-((S)-1-phenylpropyl)-2,3,6,7,8,9-hexahydro-[1,4]-dioxino[2,3-g]isoquinoline-8-ylformylamino)-3-(4-(2,3-dimethylpyridin-4-yl)phenyl)propanoic acid (“OAD2”) and a pharmaceutically acceptable salt thereof, and a preparation method therefor. The composition may contain a low level of one or more oxidative degradation substances or may contain a low level of reactive oxygen species. The pharmaceutical composition provided in the present invention has low content of oxidative degradation impurity B and a low total impurity content, so that the composition can be stably stored for 12 months or more under normal temperature and normal humidity.