Abstract: This paper studies the chronology, rock geochemistry and zircon Hf isotopic characteristics of the Middle Ordovician Cuiluan plutonic complex in the south section of the Xiaoxing'an Mountains in Heilongjiang Province, focusing on the formation age, petrogenesis, magmatic material source and tectonic background of the rock mass. Using laser ablation plasma mass spectrometer (LA−ICP-MS) zircon U−Pb dating method, the ages of granodiorite and granite porphyry samples are dated to be 463±2 Ma and 462±2 Ma, i.e., Middle Ordovician. The study of rock geochemistry shows that both samples are characterized by high Si and K, rich alkali, and poor Ti, Mg, Fe, P, etc. It is enriched in LILE (e.g., Rb, K ), depleted in HFSE (e.g., Ti, Nb, Ta, P and HREE) with significant negative Eu anomalies（δ Eu= 0.38~0.64), which show the crust source characteristics of magma. Zircon ε_Hf (t)=2.4~3.4, the values are concentrated and distributed above the chondrite line, indicating that the magma source area is the partial melting of the new continental crust material. T_DM^c = 1128~1070 Ma, confirming the existence of Mesoproterozoic crustal accretion events in this area. This study suggests that the Cuiluan complex was formed in the tectonic environment of subduction of oceanic and continental plates.