[en] We investigate the thermodynamic limit of the $su(n)$-invariant spin chain models with unparallel boundary fields. It is found that the contribution of the inhomogeneous term in the associated T–Q relation to the ground state energy does vanish in the thermodynamic limit. This fact allows us to calculate the boundary energy of the system. Taking the $su(2)$ (or the XXX) spin chain and the $su(3)$ spin chain as concrete examples, we have studied the corresponding boundary energies of the models. The method used in this paper can be generalized to study the thermodynamic properties and boundary energy of other high rank models with non-diagonal boundary fields.