Influence function is a key parameter to evaluate the capability of bimorph deformable mirrors in correcting aberrations. Influence functions of a 20-element bimorph deformable mirror are measured by Veeco interferometer and simulated by a finite element model. Compared with the measured data, simulated results come out a little smaller. But they are consistent in both shape and the location of peak. And the finite element model can predict the correction on 1～35 Zernike aberrations of the 20-element bimorph deformable mirror correctly, with errors smaller than 0.1. It is feasible to improve the bimorph deformable mirror by optimization of its finite element model.