The continuous thin-plate type with discrete actuators is widely used for active or adaptive mirrors in the medium size range from about 10cm up to 2m in diameters. The performance of a thin-plate deformable mirror could be characterized by the influence function of an actuator and the layout of the actuators. This paper first derives an equation which estimates influence functions of thin-plate deformable mirrors based on the analytic calculation and finite element analysis. Then the performance analysis for the case of equi-spaced actuators is presented.