The main function of the Visible-light Telescope (VT) in the Sino French cooperative Space Variable Objects Monitor (SVOM) satellite is to perform high-precision observation of the afterglow of a Space Gamma Burst (GRB). VT is a large aperture, long focal length telescope. Due to differences in ground and space environments, the focal plane position needs to be adjusted during orbit. In the design of VT, the focus function is achieved by adjusting the position of the correction mirror using a focusing motor. In the testing of VT, we found that the thermal elastic deformation at the base of the VT outer baffle can pull the secondary mirror supporter, and causing defocusing of the telescope, but the image quality maintains good condition.According to the main structure design of VT, kinetic analysis shows that when one end of the outer frame of the outer baffle is restricted by a constant temperature barrel assembly, and the other end is pulled by the root of the variable temperature outer baffle, the pulled end will contract inward or expand outward, and the four secondary mirror seat ribs will tilt in the same direction along the optical axis under the drive of the outer frame, driving the secondary mirror to move forward or backward in the optical axis direction. If the temperature at the base of the outer baffle is consistent in the circumferential direction, the stress applied to the outer frame of the secondary mirror supporter is circularly symmetrical and points towards the optical axis. The secondary mirror will only move along the optical axis under the driving force of the ribs, without any deviation or translation, and will not change the parallelism and coaxiality of the primary and secondary mirrors, so the system image quality will not be affected. In addition, when one end of the outer frame of the secondary mirror supporter is limited, the outer frame and ribs form an inverted “L” shaped lever structure with the limited end as the fulcrum. The ratio of the displacement of the secondary mirror to the displacement of the outer frame at one end that is pulled is approximately the ratio of the rib length to the width of the outer frame. Considering the amplification effect of the secondary mirror on the focal plane position, slight changes in the position of the secondary mirror will amplify at the focal plane, resulting in a significant defocus on the focal plane. The phenomenon of synchronous changes in the position of the secondary mirror in the optical axis direction caused by changes in the base of the outer baffle can be used as a focusing method with a larger range of travel for telescopes.In order to further analyze the characteristics of this focusing method, a thermodynamic finite element model of the outer baffle, the secondary mirror components, and the barrel assembly was established in Patran2013 software. The analysis results show that when the temperature at the root of the outer baffle changes within the range of 0 ℃~20 ℃, the relationship between the axial displacement of the secondary mirror and the root temperature of the outer baffle is 3.2 μm/℃. Due to the amplification effect of the secondary mirror, the corresponding focal plane displacement is 115 μm/℃. At the same time, simulation analysis results show that when the temperature at the root of the outer baffle is constant, the impact of temperature changes in the non root area of the outer baffle on the telescope can be ignored.In the thermal equilibrium test of VT, the relationship between the temperature at the root of the outer baffle and the defocus of the telescope was tested. Due to the non ideal characteristics of actual products, thermal equilibrium test data shows that when the temperature at the root of the outer baffle is within the range of 5 ℃ to 20 ℃, there is a good linear correspondence between the defocus amount of VT and temperature. The relationship between the defocus amount and the temperature change at the root of the outer baffle is 0.05 mm/℃, and the defocus range reaches 0.75 mm.Finally, data and experimental verification were conducted on the temperature control accuracy that meets the requirements of telescope depth of field. Based on the variation of VT defocus with the temperature at the root of the outer baffle and the optical design depth requirement of ±0.1 mm, the temperature control accuracy at the root of the outer baffle should be ±2 ℃. Assuming that the energy concentration diameter of the VT within the depth of field does not exceed 5% of the design value, the experimental data indicates that the temperature control accuracy should be ±2.1 ℃.