The invention belongs to a chamber assembly for synthesizing a diamond and cubic boron nitride sintered body and an assembling method. The chamber assembly consists of a pyrophyllite composite block, a graphite heating tube, a magnesium oxide cup, an iron cup, a graphite cup, a molybdenum cup and a niobium cup, wherein the magnesium oxide cup, the iron cup, the graphite cup, the molybdenum cup and the niobium cup assembled and placed in the graphite heating tube in the pyrophyllite composite block are in close fit, sequentially the magnesium oxide cup, the iron cup, the graphite cup, the molybdenum cup and the niobium cup are arranged from outside to inside, and the molybdenum cup and the niobium cup are fastened together in opposite directions. The materials are simultaneously put in a pyrophyllite composite block sintering chamber with the graphite heating tube in a sequence from outside to inside. The deformation of the synthesized sintered body is small, the synthesis integrity rate is high, the residual stress in a super-hard material polycrystalline layer is small, the distribution is more uniform, the porosity is low and the large-sized sintered body can be prepared.