In order to realize diagnostic instrument manipulator precise targeting, an aiming method based on visual servoing was proposed. Defined the target deviation and projection matrix in the vision system, the calculated target deviation value computed by projection matrix was closed to the true value. Using a 3DOF hybrid mechanism motion mode, the motion accuracy was promoted. With optimizing visual servoing strategy based on projection matrix and motion mode, the total targeting work was greatly simplified, making the vision system calibration unnecessary. Automatic positioning tests indicate that pointing accuracy is 11μm in X direction and 12 μm in Y direction. Radical positioning accuracy is 14 μm.Laser shot tests with streak camera show that the automatic aiming method can satisfy the requirements of project.