As the support accuracy of primary mirror of a large caliber optical telescope directly influences the overall unit of opto-electrical detecting equipment, this paper presents a support structure combined an axial support and a radial support for the primary mirror with a diameter of 1 000 mm according to the requirements of an optical system for support accuracy. Furthermore, the finite element method is used to analyze and calculate the support positions theoretically. Experiments show that when a 18-point floating support is used in the axial support and a 3-point resilient mounting in the radial support, the higher surface figure accuracy for the primary mirror can be reached. As traditional assembling and adjusting techniques can not reach the ideal surface figure accuracy, it explores a new assembling and adjusting approach to the primary mirror. An interferometer is applied to the measurement of the primary mirror with a support in real-time, then it is grinded again to obtain the higher accuracy. After the primary mirror is assembled, its surface figure accuracy reaches λ/18. The results not only meet the index requirements of the project, but also offer a new way to assemble and adjust larger caliber primary mirrors.