Thirty meter telescope tertiary mirror is one of the largest flat mirror in this word. It is also called giant steering science mirror(GSSM). In order to better analyze and constrain the jitter of GSSM, the measurement procedure shall be carefully processed. The analysis and understanding of jitter testing was necessary, for the unique and tight requirements of jitter testing. The first section was about jitter testing with high accuracy encoder. And the error was simulated by Monte Carlo method for the profile and relative location between the tracker and the GSSM was discussed. The tilt axis location error for the co-axis case was 1 μm versus the off-axis case 3 μm. When it came to the rotational axis, the location error was 4.6 μm, and the next consideration was the accelerometers used in the jitter test. For 333B32, at 2 048 Hz sampling frequency, the error was 0.6 μm above 0.05 Hz. The jitter measurement provides useful insight for the jitter of the system, the error analysis consequently specifies the statistic property of the jitter. This will also give guide to the system engineering.