The invention belongs to the technical field of spacecraft structures, and in particular relates to a spacecraft main structure having to carry a large mass storage case in the field of high orbit ordeep space exploration. The light weight-type spacecraft main structure comprises a base plate, a center angle box, a top plate, four separation plates, four side plates and four inclined side plates.The light weight-type spacecraft main structure is of an octagonal pillar structure with an outwardly-projected top part and an outwardly-projected bottom part; and the structural weight can be effectively reduced at the whole spacecraft arrangement level. To satisfy structure rigidity and equipment installation demands, the top plate is designed to be an aluminum panel aluminum honeycomb sandwich structure plate; the base plate is designed to be a carbon fiber reinforced resin stiffened shell; the rest structural plates are designed to be carbon fiber reinforced resin panel aluminum honeycomb sandwich structure plates. Instrument installation is achieved with cabin plates having the minimum area, so the whole spacecraft structural weight can be greatly reduced; and a lighter detector having a better configuration can be designed in cooperation with the overall system.