The Xiaokelehe porphyry Cu-Mo deposit in the Great Xing’an Range contains six stages of quartz-sulfide veins (V1 to V6) including potassic (V1 to V4), chlorite-illite (V5) and phyllic (V6). Two to three types of quartz were identified within each stage, of which the V2Q1, V3Q1, V4Q2, V5Q1 and V6Q2 are spatially associated with sulfides. Three types of fluid inclusions were identified in these veins, i.e., type I aqueous inclusions homogenized to liquid, type II aqueous inclusions homogenized to vapor, and type III aqueous inclusions containing halite and homogenized to liquid. Type I and type II primary inclusions occur in all stages, and type III inclusions are developed in V2 to V4 veins. Microthermometric data show that the maximum formation temperatures and pressures of V2-V6 veins of type I inclusions are 353 to 437 °C and 19.0 to 27.6 MPa, 309 to 415 °C and 15.0 to 30.1 MPa, 330 to 365 °C and 16.7 to 27.6 MPa, 243 to 351 °C and 13.9 to 18.9 MPa and 255 to 347 °C and 13.9 to 17.8 MPa, respectively, which show a decreasing trend. Oxygen isotope data show that the δ¹⁸O_H2O values of V2-V4 veins (5.3 to 10.4 ‰) are consistent with typical magmatic values, whilst the δ¹⁸O_H2O values of V5-V6 veins (0.2 to 4.6 ‰) are much lower than those of the magmatic water, which is due to involvement of meteoric water. The fluid inclusion microthermometry and O isotope data suggest an evolving magmatic-hydrothermal system with decreasing temperatures, pressures from V2 to V4 veins (potassic stage) to V5 (chlorite-illite stage) and V6 veins (phyllic stage), and an increasing incorporation of meteoric water in V5 and V6. The Xiaokelehe deposit has lower CO₂ contents than the adjacent porphyry Mo deposits formed in the same post-collisional setting, which is mainly due to differences of mineralization depths and magma source. The formation of porphyry deposits with different Mo/Cu ratios highlights the diversity and complexity of the mineralizing systems in post-collisional settings, which has important implications for mineral exploration in such environments.