The present invention relates to the technical field of medicine, and disclosed a maleate of a nicotinyl alcohol ether derivative, a crystal form thereof, and an application thereof, i.e., (S)-N-(2-(pyridin-3 -yl-methoxy)-4-(2-bromo-3 -phenylbenzyloxy)-5 -chlorobenzyl) serine isopropyl ester maleate and a stereoisomer and a crystal form thereof, a preparation method therefor, a pharmaceutical composition, and a use thereof. Specifically, the present invention relates to the (S)-N-(2-(pyridin-3 -yl-methoxy)-4-(2-bromo-3 -phenylbenzyloxy)-5 -chlorobenzyl) serine isopropyl ester maleate represented by formula I, and a crystal form thereof, a stereoisomer thereof, a preparation method therefor, a composition containing said compound or the crystal form thereof, and a use of said compound or the crystal form thereof in the preparation of a medicine for treating diseases related to a PD-1/PD-L1 signaling pathway, such as cancer, infectious diseases and autoimmune diseases.