As the foundation of quantum theory, the preparation of many-particle entangled states in macroscopic systems is one of the major goals. A spin-1/2 approximation method is proposed to study the quantum entanglement of two-component Bose-Einstein condensates (BEC) trapped in double wells. This approximation is demonstrated to be valid under the stationary tunneling condition and for odd particle numbers of each component. The evolutionary process of system states are both analyzed under effective Hamiltonian and whole Hamiltonian. Strong and long time sustained entanglement with respect to tunneling rate and the exact time can be achieved. It is discussed that the effect of the system characteristics on the entangle degree. A multibody problem which is difficult even for single component BEC can be transformed to a bipartite two-state one by this theory, similarly, the entanglement measureent can be achived more effectively.