Abstract: In order to increase the large aperture telescope third mirror supporting structure first order natural frequency, eight pre-stressing cables was used to replace the four wing beams structure. Firstly, according to the Bernoulli-Euler beam theory, the mirror supporting structure was simplified as a model composed of quality point and beam and the system natural frequency expressions was got as well as the first order natural frequency. Then, in 1.23 m telescope, with pre-stressing force at 20 000 N, the first order natural frequency comes to 18.9 Hz, comparing with the finite element software ANSYS, 17.8 Hz, with error of 6%. Finally, through the analysis of the condition that the second mirror room quality maintain invariable, relations for pre-stressing force under different primary mirror diameter and secondary mirror supporting structure of the first order modal was got. For 1.23 m telescope, pre-stressing force of 70 000 N can make the first order modal frequency reached 34 Hz. For 2 m 4 m telescope, by adjusting the pre-stressing force, the first frequency can be controlled in more than 20 Hz. The method can be used for similar structure dynamics calculation. Meanwhile, this structure can have high torsional rigidity, and can effectively reduce the weight of the secondary mirror supporting structure, the design of the optical system in the large diameter has very good guidance.