Aiming at overcoming difficulties in requirements satisfaction of the tolerance by simple connecting the optical components with their mechanical supports in the acurate-fine optical system, which means it will cost extremely expectation to the accuracy of mechanical process and adjustments,a fine tuning supporting mechanism to the 45° tilting mirror which can split or deflect the beam in optical system is presented. By employing multiple DOF tuning mechanism, it can set the 2-D tilt and take displacements in the direction normal to beam path at the same time. Flexible element is designed to improve the adjusting accuracy to the magnitude of second. Dovetail groove rail displacement technique is considered to save the cost of fund and time in machining process, and its displacement rang is 0~10 mm. Detailed analysis and discussions are presented in the key part of the mechanism by finite element analysis software. The whole research technique can be a fine reference to the optic-mechanical system of the same kinds and has great application value in beam setting of optical system.