With the development of large segmented-mirror telescopes, the segmented-mirror support technology has been researched and used widely and successfully. Besides the sub-mirror cell used to support single sub-mirror, the whole truss for supporting all the sub-mirrors also has been developed gradually. Different from the backup structure of radio telescope, the truss of optical segmented-mirror telescope needs not only huge spatial dimension but also high precision and long-term stability. In this paper, the spatial topology of truss and selection of node type have been introduced, at the same time, through the design and assembling of the primary mirror truss of LAMOST, some experiences have been also shared.