An orthogonal diagonal-placing vierendeel sandwich plate floor system unit of an assembled monolithic space steel grid box structure comprises stand columns, a transverse beam and a midspan supporting rod and further comprises ST-1, ST-2, ST-3, ST-4, ST-5, ST-6 and ST-7 double-splicing or single-splicing unit vierendeel trusses in the X2 direction, the vierendeel trusses are welded to form a multispan continuous vierendeel beam, T-shaped components are installed in the X3 direction through bolts, dowels are driven in the grid wing edges of the T-shaped components on the upper portion, formworks are erected on the grid of the T-shaped components on the upper portion and provided with rebars in a binding mode, and the X2 direction and the X3 direction respectively intersect with the column axis to form an angle of 45 degrees. The orthogonal diagonal-placing vierendeel sandwich plate floor system unit solves the problem the transporting outlet width is limited, on-site assembly is performed without high-strength bolts so that welding joints can be substantially reduced, and the effect of saving cost is achieved.