To realize active optics and further adaptive optics, high precision positioning technology with nanometer's resolution is believed to play a vitally important role in driving segmented mirrors of very large astronomical telescope (VLAT). The positioning actuators based on hydraulic technology, flexible-hinge principle and motor-reducer approach have been generally employed in VLAT to date. But such actuators prove to have the drawbacks such as hydraulic leakage, lagging, backlash and insufficient stroke, which could hardly be avoided in application of the above mentioned technologies. Naturally, these actuators can not satisfy the high-precise-positioning requirement for active and adaptive optics in VLAT. Aiming at upgrading the performance of actuators in VLAT, this paper presents the conceptual design of a small giant-magnetostrictive actuator which features stable nano-resolution positioning, large stroke, and instant response, high load capacity and free from backlash. Based on inchworm moving principle, the current research focuses on developing the elongating mechanism for longer step displacement with nanometric resolution and the clamping mechanism for large driving force. The feasibility of attaining a target actuator with 1000 microstrains and 4 nanometers resolution has been promised by relative experiments upon a Φ7×20 mm Terfenol-D rod in the authors' previous researches. Besides, the design of clamping mechanism as the other focus adopts magnetostrictive method to generate remarkable clamping force and to enable self-locking/pre-clamping behavior as well, which the other clamping methods are most unlikely to achieve. The current research is expected to contribute to meet the needs of high precision positioning for active and adaptive optics in VLAT.