On the basis of summing up the difficulties of testing large flat mirror, for the sake of testing super-large surface especially for the tertiary mirror of Thirty Meter Telescope (TMT-M3) with precise method, a new technology syncretized pentaprism scanning method and Subaperture Stitching Interferometry (SSI) was proposed. The large mirror was tested by pentaprism scanning and SSI successively, the basic principle of the metrology was introduced and researched, and a reasonable optical testing project of TMT-M3 with the aperture of 3.5 m×2.5 m was made, the flow chart, pentaprism design, aberration fitting of pentaprism scanning, and the optimum stitching algorithm were analyzed particularly. And a prototype of TMT-M3 was developed for experimental verification. The Root Mean Square (RMS) and slope Root Mean Square (slopeRMS) values are 28.676 nm and 0.97 μrad, respectively.