This paper proposes research of a structure of LAMOST fiber positioning medium-term system; its construction and its tests are briefly introduced. This medium-term system includes several parts as follow: a main control computer, a unit controller, a set of drive circuits, 19 optical positioning units that positioning 19 optical fiber ends on the small simulate focal plane with diameter of 180 mm, a CCD camera, a frame grabber, and control programs. Tests on the system have indicated that positioning precision of 19 units is less than 0.04 mm on the whole focal plane with diameter of 180 mm. On medium-term system, some important problem for LAMOST building has test and research, for example: Fiber positioning precision, mechanism interference among the units, anti-jamming of drive circuits, unit's work life-span and reliability, temperature raising, etc. Test results have established stability foundation for LAMOST construction.