The present invention relates to the technical field of medicine, and specifically relates to a compound represented by formula I, a pharmaceutically-acceptable salt and stereoisomer thereof. R4, R4' , R5, R5' , R6, R7, R1a, R2a, m, n, t1, and t2 are as defined in the description. The present invention further relates to a crystal form of the compound represented by formula I, and a pharmaceutical preparation and pharmaceutical composition thereof, a preparation method therefor, and a use thereof in preparing a drug for treating relevant diseases mediated by an indoleamine 2, 3-dioxygenase (IDO) anomaly. (I)