  In $\phi^6$ theory, the resonance scattering structure is triggered by the
so-calls delocalized modes trapped between the $\bar{K}K$ pair. The frequencies
and configurations of such modes depend on the $\bar{K}K$ half-separation 2$a$,
can be derived from the Schr\"{o}dinger-like equation. We propose to use the
periodic boundary conditions to connect the localized and delocalized modes,
and use periodic boundary approximation (PBA) to solve the spectrum
analytically. In detail, we derive the explicit form of frequencies,
configurations and spectral wall locations of the delocalized modes. We test
the analytical prediction with the numerical simulation of the
Schr\"{o}dinger-like equation, and obtain astonishing agreement between them at
the long separation regime.
