Abstract: The slope methods was introduced to measure the TMT M3 mirror figure. First of all,the low order aberrant and middle spatial frequency aberrant were studied for the search of property plan to test and analyse them. Then the problem was considered numerically for the prove of theory, furthermore, the experiment was also done to confirm the feasibility of this plan. Lastly, for the error budget of this system, the Monte Carlo method was introduced to allocate the error to the instrument of the sub-system for request of TMT. The tertiary mirror of TMT and other reflecting mirror frequency feature, and quantification could be measured. As a result, it could give more sufficient technical guidance on the testing for the large aperture reflect mirror.