In order to make the adaptive optical system function well, it needs to have a full understanding of the performance of the deformable mirror, so a technique for the flatten calibration of the deformable mirror based on the measurement of the interferometer is presented. Firstly, all the PZT actuators are set to the midrange of their control voltage; and then, the height of the mirror surface with the ZYGO interferometer is measured, the height of mirror surface at each actuator is calculated, and the differences between the height of the mirror surface at each actuator and the average height of the mirror surface are figured out; finally, the actuators are controlled to reduce the differences to zero. The results show that affected by the hysteresis of the PZT actuator, the mirror surface needs 4 to 6 iterative processes showed above to reach the same preset desired flatness; for the 21elements deformable mirror, which without the actuators under the periphery of the mirror, an rms surface figure near λ/20（λ=632.8 nm） at its 80% aperture is achieved; for the 137elements deformable mirror, which has actuators under the periphery, the rms surface figure is better than λ/50. Under different observation conditions of the telescope, this technique can flat the deformable mirror rapidly in the adaptive optical system, in order to adapt to changing working conditions.