We investigate the thermodynamic limit of the inhomogeneous T − Q relation of the antiferromagnetic XXZ spin chain with antiperiodic boundary condition. It is shown that the contribution of the inhomogeneous term for the ground state can be neglected when the system-size N tends to infinity, which enables us to reduce the inhomogeneous Bethe ansatz equations (BAEs) to the homogeneous ones. Then the quantum numbers at the ground states are obtained, by which the system with arbitrary size can be studied. We also calculate the twisted boundary energy of the system. the contribution of the inhomogeneous term with finite system-size N . We find that the contribution of the inhomogeneous term in the associated T − Q relation to the ground state energy can be neglected when the system-size N tends to infinity. Because we consider the massive region of the system, the ground state energy with even N and that with odd N are different. The value of energy difference is proportional to the energy of one bond. We also check our results by using the density matrix renormalization group (DMRG) method [23,24], which leads to that the numerical results and the analytic one are consistent with each other very well. As a consequence, we obtain the twisted boundary energy of the model. The paper is organized as follows. In the next section, the model and the associated ODBA solutions are introduced. In section 3, we study the finite-size effects of contribution of the inhomogeneous term in the T − Q relation for the ground state. The thermodynamic limit of the XXZ spin chain with antiperiodic and with periodic boundary conditions are discussed in section 4 and section 5, respectively. The twisted boundary energy is given in Section 6. Section 7 is the concluding remarks and discussions. Some supporting detailed calculations are given in Appendices A&B.