Active optics is one of the key technologies for constructing modern large telescope. It's necessary to study the performance of an active optics system theoretically in advance. In this paper we built a 1.2 m active thin-mirror model with 36 axial supports and 3 lateral supports. In order to ensure the active optics system and select appropriate force actuators, we carried out following analyses through FEA: the ability of the active optics system to duplicate the first 15 Zernike modes and the first 15 natural resonance modes; mirror deformations before and after active force corrections under gravitational loads; the maximum stresses of the mirror while fitting Zernike modes of 1 micron PV amplitude; requirements of the force accuracy and the maximum load for the force actuators. Through above analyses over the theoretical performance of the 1.2 m active thin-mirror, it can be concluded that: the support system is effective to maintain the mirror surface; the maximum stresses of the mirror while fitting aberrations are within the allowable stress of the glass material; the force resolution is about 0.5 N and the maximum load for the force actuators is about 400 N. As a result fundamentals are set up for constructing a practical active mirror.
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