How the deformation of amphiboles and their involvement in metamorphic reactions during shearing affect the rheology of the middle crust remains controversial. Middle crustal fault rocks are exposed along the Shuiyu shear zone (SYSZ) in Yunmeng Shan, Northern Beijing, which was formed by the exhumation of the Yunmeng Shan metamorphic core complex during the Early Cretaceous tectonic extension of the North China Craton. Various deformation structures, optical microstructures, dislocation submicrostructures and fabrics during the transition from brittle to ductile deformation in the middle crust are observed in the sheared diorites. In particularly, two types of amphiboles, i.e., the A-type with brown cores and green mantles as porphyroclasts and the B-type of green-colored fine grains in the matrix, distributed biotite grains and interconnected fine biotite zones, are observed in the deformed diorites. Geothermometer analysis reveals that the amphibole grains in the sheared diorites were crystallized at temperatures over c. 720 °C, deformed at c. 680 °C–600 °C and transformed into biotite at c. 600 °C–520 °C. During these processes, fluid not only enhanced crystal plasticity and governed the deformation of the amphibole grains, but was also involved in their metamorphic transformation into biotite. Our results show that the diorites along the Shuiyu shear zone experienced three stages of middle crustal shearing and weakening, i.e., an early stage of fluid-enhanced crystal plasticity followed by rigid body rotation, a middle stage of transformation from a strong phase (amphibole) to a weak phase (fine-grained biotite and quartz) and a late stage of weak phase interconnection.