A 961-element deformable mirror (DM) and its control system are developed to satisfy the demand of the adaptive optical system of the astronomical telescope. The performance of the deformable mirror is tested by Zygo interferometer. Three tests are carried out, which include the response function test of individual actuator, the push-pull capability test of the neighbor actuators and the coupling test between two actuators. Then, the correction capability of the deformable mirror is tested by performing the Waffle mode test, Zernike polynomial fitting test and flattening test. The testing results show that the max correction ability of the deformable mirror actuated by individual actuator is ±2.5 μm. The inter-actuator stroke is 4 μm. And the coupling between two neighbor actuators is 23%. Compared to the former 137-element deformable mirror, the fitting capability of the Zernike polynomials of the 961-element deformable mirror is much better. The surface flatness of the deformable mirror after flattening is better than root-meat-square (RMS) value of λ/70 (λ=632.8 nm). The test results satisfy the requirements of the deformable mirror which is to be used in the adaptive optical system of the astronomical telescope.