In large-aperture telescopes， the relative positions of the primary and secondary mirrors must conform to stringent specifications. The secondary-mirror system is frequently designed as an adjustable mechanism with several degrees of freedom， owing to the high quality of the primary mirror. This significantly affects the telescope imaging. The secondary-mirror truss and adjustment mechanism are combined and designed to decrease the overall height of the telescope. Moreover， a secondary-mirror truss-adjustment mechanism that can be employed in large-diameter telescopes is designed. First， a detailed description of the designed adjustment mechanism is given， followed by static and modal analyses， and finally， a kinematic-performance test of the experimental prototype. The moving stroke of the designed mechanism in the Z direction can reach ±5 mm and the absolute positioning accuracy is better than 16 μm. The deflection stroke in the X/Y direction can reach ±0.574° and the absolute positioning accuracy is better than 6.4″. The mechanism complies with the itinerary requirements and secondary-mirror adjustment accuracy for large-diameter telescopes.