A non-fusuline foraminiferal fauna consisting of 20 genera and 27 species was documented from the middle part of the Xiala Formation at the Mujiucuo section of Xainza County, central Tibet. This fauna consists mostly of porcelaneous-walled genera such as Agathammina, Midiella, Hemigordiopsis, Neodiscus, Multidiscus, and a new genus Megacrassispirella Zhang gen. nov. These taxa indicate a Midian (Capitanian) age. The climatic amelioration, triggered by climatic warming after the Late Paleozoic Ice Age or by northern drift of the Lhasa Block, promoted diversification of the non-fusuline foraminifers in the middle part of the Xiala Formation. Warm oceanic currents also played an important role in bringing biota from low latitude areas to the northern Gondwanaland margin through the newly-formed Neotethys Ocean. One new genus, Megacrassispirella gen. nov., with Megacrassispirella xarlashanensis (Wang, 1986) emend. Zhang as type species, and one new species, Agathammina vachardi Zhang sp. nov., are described.