Most aspheric mirrors have been measured by null lens test or computer generated hologram method. This approach, however, requires that the surface be close to the target shape; otherwise, the testing may not be possible or correct. Hartmann test has an advantage in that it has larger dynamic range than the general interferometer, which means that the surface can be measured with a Hartmann test from the early stage of polishing process. In this paper, authors show the measurement capability of Hartmann sensor, compared to the ZYGO phase-shifting interferometer and implement the Hartmann test to the measurement of 0.9 m aspheric mirror from the beginning of the polishing process. In order to increase the measurement accuracy and reduce the measurement time, authors develop the special null optics. The early stage of aspheric mirror was measured with the inteferometer and Hartmann sensor together, to show the great usefulness of Hartmann test in the measurement of large wavefront error, which can in turn reduce the manufacturing time of large optics.