Abstract: In order to complete the testing and alignment of TMT third mirror shafting, the maximum likelihood estimation was introduced. Firstly, two intersecting planes were used to identify a space line. Then, considering the noise of the measured data, maximum likelihood estimation was made use of to estimate TMT third mirror shafting parameters. And in MATLAB, which produced a training set with Gaussian white noise, the angle of collection axis and ideal axis from 6.29 to the optimized 5.24 was reduced, with optimization of 17%. Lastly, Vantage Laser Tracker was made the testing tool for TMT large shafting. Using optimization before, the TMT third mirror shafting residuals error was drawn to 2.9, which was less than the TMT indicator of 4. This paper will do good to TMT third mirror shafting alignment, and raise a real-time method to other large diameter optical system shafting alignment.