The mafic-ultramafic rocks of Dongdougou are located in the orogenic belt between the Tarim plate and the Yili-central Tianshan plate, southwestern Tianshan. Harzburgite is the main type of peridotite, generally metasomatized by hydrous melt or fluid. It includes different types of spinels, being red-brown, brown, dark green, and light green in color. Cr₂O₃ and FeO decrease gradually whereas Al₂O₃ and MgO increase with the metasomatism. Studies indicate that red-brown and brown picotiles constitute the primary type, while green magnesium spinels possess the second place. The compositions of the primary picotiles are variable with Cr^# and Mg^# in the range of 0.41～0.87 and 0.21～0.61 respectively, which may indicate polyphase origin of melting. Secondary spinels have different shapes, and their compositions vary tremendously, exhibiting two stages of metasomatism or metamorphism. The primary picotiles have characteristics of peridotite in a subducting upper plate mantle wedge zone, which implies the host rocks might have been the fragments of peridotite from a SSZ volcanic arc environment.