The key factor for developing cable plastic cross-linked polyethylene cable is to eliminate space charge in the bulk. Nowadays, it is universally received that the suppression mechanism of charge accumulation in polyethylene/nano-particle composite is the formation of deep traps for trapping charges, which, in fact, is contrary to the principles of electrical field. So in this paper, the formation and the suppression mechanisms of space charge are elaborated by the energy band theory of polymeric dielectric. Then based on the first order trap model, the formation of space charge in polymeric dielectric is deduced by dynamical equation of the trapped and detraped charges. When the deep traps are introduced into polymeric dielectric, a displacement of Fermi energy level in dielectric occurs and the electric contact of interface between electrode and dielectric changes from ohmic contact to blocking contact. The width of the depletion region associated with blocking contact is less than 100 , due to huge density of traps existing in amorphous polyethylene (PE). The tunnel effect of electron makes the electrical contact of interface a neutral contact. The space charges cannot be formed in PE dielectric under electrical stress. Finally, the conductive current as a function of electrical stress and the space charge distribution are measured respectively on both PE samples, one is pure PE and the other is the nano-particle modified PE filled with deep traps. The test results are consistent with the theoretical results.