In order to improve imaging quality of the space camera, a thermally driven focusing structure is designed to compensate the offset. Firstly, the focusing method is chosen according to the optical system, and the focusing structure is designed by driving the secondary mirror to move along the direction of the optical axis which occupies small space and is light in quality. Then, according to the demand of focusing, the aluminum with higher thermal expansion coefficient is selected as the main material. To ensure the secondary mirror works at constant temperature, some heating films are attached to the back of the mirror seat. And the performance of the focusing structure is simulated by the finite element method. Finally, the working state of the focusing structure is analyzed, and the deformation of the mirror caused by the temperature change is fitted by Zernike polynomial, and the fitting coefficients are input into the Zemax. As the measurement of imaging quality, the modulation transfer function is used to reflect the impact of the deformation of the sub-mirror on the optical performance of the space camera. The result shows that the focusing structure meets the design specifications and the impact of deformation belonging to the sub-mirror on the imaging quality of optical system can be neglected when the focusing structure is working. Compared with traditional focusing structures, the thermally driven focusing structure has the advantages of light weight and simple structure.