  We perform numerical simulations to study the secular orbital evolution and
dynamical structure in the HD 69830 system with the best-fit orbital solutions
by Lovis and coworkers (2006). In the simulations, we show that the triplet
Neptunian system can be stable at least for 2 Gyr and the stability would not
be greatly influenced even if we vary the planetary masses. In addition, we
employ the Laplace-Lagrange secular theory to investigate the long-term
behaviors of the system, and the outcomes demonstrate that this theory can well
describe the secular orbital evolution for all planets, where the secular
periods and amplitudes in the eccentricities well agrees with those of the
direct numerical integrations. We first reveal that the secular periods of the
eccentricity $e_{1}$ and $e_{2}$ are identical about 8,300 yr. Moreover, we
explore the planetary configuration of three Neptune-mass companions with one
massive terrestrial planet in 0.07 AU $\leq a \leq 1.20$ AU, to examine the
asteroid structure in this system. We underline that there are stable zones at
least $10^{5}$ yr for low-mass terrestrial planets locating between 0.3 and 0.5
AU, and 0.8 and 1.2 AU with final low eccentricities. Still, we also find that
the secular resonance $\nu_{1}$ and $\nu_{2}$ of two inner planets can excite
the eccentricities of the terrestrial bodies, and the accumulation or depletion
of the asteroid belt are also shaped by orbital resonances of the outer
planets, i.e., 5:2 and 1:2 MMRs with Planet D... (abridged)
