Extensive Phanerozoic granitoids in northeastern China have constituted one of the hotspots in the study of granite in the eastern Central Asian Orogenic Belt. In particular, the petrogenesis and tectonic affinity of granites in the Xiao Hinggan Mountains-Zhangguangcai Mountains have always been controversial. In this study, zircon LA-ICP-MS U-Pb dating of the monzogranites from the Tieli and Yilan areas in the Xiao Hinggan Mountains-Zhangguangcai Mountains yielded crystallization ages of 188±1 Ma and 257±3 Ma, respectively. The geochemical data show that these monzogranites belong to I-type granite and are characterized by enrichment of LILE (e.g., Th and U) and LREE and depletion of HFSE (e.g., Nb and Ta) and HREE, which are typical for continental arc-type granites. Zircon Hf isotopic data indicate that the granitic magma in the Tieli area was derived from partial melting of Meso-to Neoproterozoic lower crust. Combined with previously published geochronological and geochemical data, it is inferred that the Xiao Hinggan Mountains-Zhangguangcai Mountains was under an active continental margin setting during the Late Paleozoic to Mesozoic. Moreover, the Late Paleozoic to Mesozoic granitoids show a westward increasingly younger trend in the whole Xiao Hinggan Mountains-Zhangguangcai Mountains. The authors hold that this temporal and spatial relationship resulted from the flattening westward subduction of the Mudanjiang Ocean between the Jiamusi and Songnen blocks.