To design an attitude adjusting mechanism for large lenses, the adjusting characteristics of a three-pod adjusting structure and its effect on the force and deformation were analyzed. On the basis of the geometrical method, the relationship between the length of each pod and the plate angle variation was obtained under the arbitrary triangle layout of 3-pod adjusting structure,and the correlation coefficient was thus defined. Furthermore,the Finite Element Method(FEM) was used to analyse the deformation of the adjusting plate with three types of support layouts. The results show that the adjusting values of 3-pod mechanism can be calculated.For a plate with the size of 580 mm×570 mm×20 mm, the maximum deformation caused by gravity could be 12.51 μm while the isoscele triangle support layout is adopted. Finally,the attitude adjusting mechanism based on the 3-pod isoscele triangle support layout was calibrated and adjusted,the results prove that it is effective in attitude adjusting and accuracy stability in target-firing experiments.