Maocaoping is a newly-discovered vein Cu deposit in western Lanping Basin of western Yunnan. The deposit contains abundant hydrothermal tourmaline, which has not been found in the other vein Cu deposits in this region. The authors investigated the characteristics of the tourmaline and analyzed its chemical composition so as to understand the evolution of the hydrothermal system, the factors controlling tourmaline growth and composition, and the ore-forming fluid sources. Mineralization at Maocaoping consists of Cu-bearing quartz-car_bonate-sulfide veins and associated Cu-barren hydrothermal alteration halos. Two types of tourmaline have been identified: ① tourmaline in the alteration halos (T1), which was formed relatively early; ② tourmaline in the veins (T2), which was formed relatively late. T1 tourmaline is anhedral, fine-grained, and aligned parallel to the mylonitic foliation of host rocks. It can be divided into two subtypes: metaclastic-hosted tourmaline (T1-Q) and marble-hosted tourmaline (T1-M). T2 tourmaline is euhedral, coarse-grained, and sub-parallel to the mylonitic foliation of host rocks or grows along or adjacent to the boundaries as comb tourmaline aggregates. The growth characteristics of T1 and T2 tourmaline suggest that the shear stress during the formation of tourmaline at Maocaoping was evolving from strong shearing through weak shearing to no shearing. Electron microprobe analyses of the tourmaline at Maocaoping show that all the tourmaline belongs to alkali group and dravite-schorl series, with dravite being dominant. T2 tourmaline has oscillatory zoning in back-scattered electron (BSE) images, suggesting their growth in an environment where physical and chemical properties fluctuated. The composition of tourmaline at Maocaoping is mainly controlled by the composition of the fluid phase. However, T1-Q tourmaline is slightly enriched in Al, and T1-M tourmaline is slightly enriched in Ca, suggesting that their compositions were slightly influenced by their host rocks, respectively. Tourmaline in Maocaoping is mainly dravitic and falls in 4, 5, 6 regions of the Al-Fe-Mg diagram, implying that the ore-forming fluid could not be derived from magmatic water of S-type granite with no interaction with the pathway rocks of fluid. The significantly higher Fe³⁺/(Fe³⁺+Fe²⁺) ratios in T2 tourmaline than in T1 tourmaline imply that the pre-ore stage had relatively low oxygen fugacity than the ore stage. These features, combined with the characteristics of ore-forming fluid and alteration, metal assemblages, and the lithology of host rocks in the other vein Cu deposits in western Lanping Basin, suggest that a reduced, CO₂_buffered pH value fluid (reduced sulfur>oxidized sulfur) was responsible for Cu transportation in the vein Cu deposits, and increasing oxygen fugacity of fluid might have played an important role in the Cu precipitation.