A closed-loop adaptive optics system, which consists of a 20-element bimorph deformable mirror and a Shack-Hartmann wave-front sensor with 13×13 sub-aperture arrays, is established to test the spatial fitting capability to 3～20 Zernike items of the bimorph deformable mirror. Constrast is made between experimental result and simulation result. And the effect of mismatch between bimorph deformable mirror and the sub-apertures of Shack-Hartmann sensor is analyzed. Results show that most of the fitting errors of 3～20 Zernike items are smaller than 0.5, and the mean difference between experiment and simulation is smaller than 0.1. Compared with rigorous aligned ideal situation, the fitting error gets lager with the increase of mismatch size. The effect of mismatch is more serious for higher order aberrations than the lower ones.